url/base(fix[doctest]): unregister demo rule after example

why: The RuleMap.register doctest added 'gl-prefix' to the global
GitURL.rule_map without removing it, unlike the git/svn/hg examples
which clean up. The leaked rule persisted into later tests.

what:
- Unregister 'gl-prefix' at the end of the example

Author: tony

src/libvcs/url/base.py

@@ -178,6 +178,10 @@ def register(self, cls: Rule) -> None:
         >>> GitURL.is_valid(url='gitlab:vcs-python/libvcs')
         True
 
+        Clean up so the examples below see the default rules:
+
+        >>> GitURL.rule_map.unregister('gl-prefix')
+
         **Example: git URLs + pip-style git URLs:**
 
         This is already in :class:`GitURL` via :data:`PIP_DEFAULT_RULES`. For the