UI: Add Title to Extensions Repo Popover

Just adds a "Repositories" title to the repo popover in
Properties / Extensions.

Pull Request: https://projects.blender.org/blender/blender/pulls/119009

scripts/startup/bl_ui/space_userpref.py

@@ -2075,6 +2075,8 @@ class USERPREF_PT_extensions_repos(Panel):
         paths = context.preferences.filepaths
         active_repo_index = paths.active_extension_repo
 
+        layout.label(text="Repositories")
+
         row = layout.row()
 
         row.template_list(