Fix Fixed fixed to Fixes (#13)

The headings are in the comment at the top, but "Fixes" is not one of
them, instead it's "Fixed". I presume that I said the wrong thing in
#11.

Author: DJMcNab

CHANGELOG.md

@@ -19,7 +19,7 @@ This release has an [MSRV][] of 1.70.
 
 This release has an [MSRV][] of 1.70.
 
-### Fixes
+### Fixed
 
 - Allow compilation to targets without 64-bit atomics. ([#11][] by [@nicoburns][])