Make it explicitly clear that `dev-` shouldn't be in the actual branch name. (#10335)

doc/05-repositories.md

@@ -342,8 +342,9 @@ project to use the patched version. If the library is on GitHub (this is the
 case most of the time), you can fork it there and push your changes to
 your fork. After that you update the project's `composer.json`. All you have
 to do is add your fork as a repository and update the version constraint to
-point to your custom branch. In `composer.json`, you should prefix your custom
-branch name with `"dev-"`. For version constraint naming conventions see
+point to your custom branch. In `composer.json` only, you should prefix your
+custom branch name with `"dev-"` (without making it part of the actual branch
+name). For version constraint naming conventions see
 [Libraries](02-libraries.md) for more information.
 
 Example assuming you patched monolog to fix a bug in the `bugfix` branch: