Update src/building/compiler-documenting.md

Co-authored-by: Yuki Okushi <[email protected]>

Author: tshepang

Diff for: src/doc/rustc-dev-guide/src/building/compiler-documenting.md

@@ -36,7 +36,7 @@ A more conventional way is using `/x.py doc`.
 - Document internal rustc items
 
   Compiler documentation is not built by default.
-  To enable it, modify "config.toml":
+  To enable it, modify `config.toml`:
 
   ```toml
   [build]