Fix IndexObject.abspath docstring formatting

The original problem where the backslash wasn't included in the
docstring at all was fixed in 7dd2095 (gitpython-developers#1725), but the backslash
still did not appear in rendered Sphinx documentation, because it
was also treated as a reStructuredText metacharacter.

Although that can be addressed by adding a further backslash to
escape it, the effect is ambiguous when the docstring is read in
the code. So this just encloses it in a double-backticked code
span instead, which is a somewhat clearer way to show it anyway.

git/objects/base.py

@@ -226,7 +226,8 @@ def abspath(self) -> PathLike:
             Absolute path to this index object in the file system (as opposed to the
             :attr:`path` field which is a path relative to the git repository).
 
-            The returned path will be native to the system and contains '\' on Windows.
+            The returned path will be native to the system and contains ``\`` on
+            Windows.
         """
         if self.repo.working_tree_dir is not None:
             return join_path_native(self.repo.working_tree_dir, self.path)