Fix intra-doc link warnings

[GuillaumeGomez]

When cargo doc -Zunstable-options -Zrustdoc-scrape-examples (trying to figure out why it doesn't work with bevy), I had the following warnings:

warning: unresolved link to `Quad`
 --> examples/2d/mesh2d.rs:1:66
  |
1 | //! Shows how to render a polygonal [`Mesh`], generated from a [`Quad`] primitive, in a 2D scene.
  |                                                                  ^^^^ no item named `Quad` in scope
  |
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
  = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: `bevy` (example "mesh2d") generated 1 warning
warning: unresolved link to `update_weights`
 --> examples/animation/morph_targets.rs:6:17
  |
6 | //!   See the [`update_weights`] system for details.
  |                 ^^^^^^^^^^^^^^ no item named `update_weights` in scope
  |
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
  = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: public documentation for `morph_targets` links to private item `name_morphs`
 --> examples/animation/morph_targets.rs:7:43
  |
7 | //! - How to read morph target names in [`name_morphs`].
  |                                           ^^^^^^^^^^^ this item is private
  |
  = note: this link will resolve properly if you pass `--document-private-items`
  = note: `#[warn(rustdoc::private_intra_doc_links)]` on by default

warning: public documentation for `morph_targets` links to private item `setup_animations`
 --> examples/animation/morph_targets.rs:8:48
  |
8 | //! - How to play morph target animations in [`setup_animations`].
  |                                                ^^^^^^^^^^^^^^^^ this item is private
  |
  = note: this link will resolve properly if you pass `--document-private-items`

warning: `bevy` (example "morph_targets") generated 3 warnings
warning: unresolved link to `Quad`
 --> examples/2d/mesh2d_vertex_color_texture.rs:1:66
  |
1 | //! Shows how to render a polygonal [`Mesh`], generated from a [`Quad`] primitive, in a 2D scene.
  |                                                                  ^^^^ no item named `Quad` in scope
  |
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
  = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: `bevy` (example "mesh2d_vertex_color_texture") generated 1 warning
warning: unresolved link to `UIScale`
 --> examples/ui/ui_scaling.rs:1:36
  |
1 | //! This example illustrates the [`UIScale`] resource from `bevy_ui`.
  |                                    ^^^^^^^ no item named `UIScale` in scope
  |
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
  = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: `bevy` (example "ui_scaling") generated 1 warning
warning: unresolved link to `dependencies`
 --> examples/app/headless.rs:5:6
  |
5 | //! [dependencies]
  |      ^^^^^^^^^^^^ no item named `dependencies` in scope
  |
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
  = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: `bevy` (example "headless") generated 1 warning
warning: unresolved link to `Material2d`
 --> examples/2d/mesh2d_manual.rs:3:26
  |
3 | //! It doesn't use the [`Material2d`] abstraction, but changes the vertex buffer to include verte...
  |                          ^^^^^^^^^^ no item named `Material2d` in scope
  |
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
  = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: `bevy` (example "mesh2d_manual") generated 1 warning