docs(pull_mirror): fix incorrect start() method usage example

The documentation incorrectly showed calling start() on the
ProjectPullMirror object (mirror.start()), but the method only
exists on the ProjectPullMirrorManager. Updated the example to
show the correct usage: project.pull_mirror.start().

Fixes python-gitlab#3269

Author: JohnVillalovos

docs/gl_objects/pull_mirror.rst

@@ -33,6 +33,6 @@ Update an existing remote mirror's attributes::
     mirror.only_protected_branches = True
     mirror.save()
 
-Start an sync of the pull mirror::
+Start a sync of the pull mirror::
 
-  mirror.start()
+  project.pull_mirror.start()