Harmonize title formatting in docs/content/doc/development/api-usage.en-us.md (#24529)

Author: Benjamin-Loison

docs/content/doc/development/api-usage.en-us.md

@@ -119,7 +119,7 @@ curl -v "http://localhost/api/v1/repos/search?limit=1"
 < x-total-count: 5252
 ```
 
-## API Guide:
+## API Guide
 
 API Reference guide is auto-generated by swagger and available on:
 `https://gitea.your.host/api/swagger`