Fix inconsistent heading level in the rustdoc book

Author: camelid

src/doc/rustdoc/src/unstable-features.md

@@ -349,7 +349,7 @@ Some methodology notes about what rustdoc counts in this metric:
 Public items that are not documented can be seen with the built-in `missing_docs` lint. Private
 items that are not documented can be seen with Clippy's `missing_docs_in_private_items` lint.
 
-## `-w`/`--output-format`: output format
+### `-w`/`--output-format`: output format
 
 When using
 [`--show-coverage`](https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#--show-coverage-get-statistics-about-code-documentation-coverage),