include missing submodule on bootstrap

As of #125408 PR,
rustbook now relies on dependencies from the "src/doc/book" submodule.

However, bootstrap does not automatically sync this submodule before reading
metadata informations. And if the submodule is not present, reading
metadata will fail because rustbook's dependencies will be missing.

This change makes "src/doc/book" to be fetched/synced automatically
before trying to read metadata.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/lib.rs

@@ -469,7 +469,8 @@ impl Build {
 
             // Make sure we update these before gathering metadata so we don't get an error about missing
             // Cargo.toml files.
-            let rust_submodules = ["src/tools/cargo", "library/backtrace", "library/stdarch"];
+            let rust_submodules =
+                ["src/tools/cargo", "src/doc/book", "library/backtrace", "library/stdarch"];
             for s in rust_submodules {
                 build.update_submodule(Path::new(s));
             }