docs: Use correct cargo-flamegraph upstream repo URL (bevyengine#6873)

# Objective

Links to `cargo-flamegraph`'s repo point to a [fork](https://github.com/killercup/cargo-flamegraph), not the actual upstream repo. We should point to the source of truth instead of a fork that hasn't been updated since 2019.

## Solution

Change links to point to the upstream repo at  [flamegraph-rs/flamegraph](https://github.com/flamegraph-rs/flamegraph).

docs/profiling.md

@@ -78,7 +78,7 @@ For more details, check out the [tracing span docs](https://docs.rs/tracing/*/tr
 
 This approach requires no extra instrumentation and shows finer-grained flame graphs of actual code call trees. This is useful when you want to identify the specific function of a "hot spot". The downside is that it has higher overhead, so your app will run slower than it normally does.
 
-Install [cargo-flamegraph](https://github.com/killercup/cargo-flamegraph), [enable debug symbols in your release build](https://github.com/killercup/cargo-flamegraph#improving-output-when-running-with---release), then run your app using one of the following commands. Note that `cargo-flamegraph` forwards arguments to cargo. You should treat the `cargo-flamegraph` command as a replacement for `cargo run --release`. The commands below include `--example EXAMPLE_NAME` to illustrate, but you can remove those arguments in favor of whatever you use to run your app:
+Install [cargo-flamegraph](https://github.com/flamegraph-rs/flamegraph), [enable debug symbols in your release build](https://github.com/flamegraph-rs/flamegraph#improving-output-when-running-with---release), then run your app using one of the following commands. Note that `cargo-flamegraph` forwards arguments to cargo. You should treat the `cargo-flamegraph` command as a replacement for `cargo run --release`. The commands below include `--example EXAMPLE_NAME` to illustrate, but you can remove those arguments in favor of whatever you use to run your app:
 
 * Graph-Like Flame Graph: `RUSTFLAGS='-C force-frame-pointers=y' cargo flamegraph -c "record -g" --example EXAMPLE_NAME`
 * Flat-ish Flame Graph: `RUSTFLAGS='-C force-frame-pointers=y' cargo flamegraph --example EXAMPLE_NAME`