Fix broken formatting for ### Library header (#6910)

This formatting looks correct in a GitHub preview, but the renderer used by the actual website shows this as unformatted text under the bullet point.

This commit fixes the formatting to be consistent with other headers.

Author: zealousidealroll

content/2025-08-13-this-week-in-rust.md

@@ -143,7 +143,8 @@ If you are an event organizer hoping to expand the reach of your event, please s
 * [preserve the `.debug_gdb_scripts` section](https://github.com/rust-lang/rust/pull/143679)
 * [simplify dead code lint](https://github.com/rust-lang/rust/pull/144863)
 * [upgrade `semicolon_in_expressions_from_macros` from warn to deny](https://github.com/rust-lang/rust/pull/144369)
- #### Library
+
+#### Library
 * [stabilize `duration_constructors_lite` feature](https://github.com/rust-lang/rust/pull/145135)
 * [stabilize `panic_payload_as_str` feature](https://github.com/rust-lang/rust/pull/144861)
 * [stabilize `strict_overflow_ops`](https://github.com/rust-lang/rust/pull/144682)