Fix recent inconsistency, using :raise:, not :raises:

GitPython used :raise: consistently before, but I recently
introduced some occurrences of :raises: by accident. This changes
those to :raise:.

git/index/fun.py

@@ -88,7 +88,7 @@ def run_commit_hook(name: str, index: "IndexFile", *args: str) -> None:
     :param args:
         Arguments passed to hook file.
 
-    :raises HookExecutionError:
+    :raise HookExecutionError:
     """
     hp = hook_path(name, index.repo.git_dir)
     if not os.access(hp, os.X_OK):

git/repo/base.py

@@ -784,7 +784,7 @@ def merge_base(self, *rev: TBD, **kwargs: Any) -> List[Union[Commit_ish, None]]:
             :class:`~git.objects.commit.Commit`, or is empty if no common merge base
             exists.
 
-        :raises ValueError:
+        :raise ValueError:
             If fewer than two revisions are provided.
         """
         if len(rev) < 2:

git/repo/fun.py

@@ -59,7 +59,7 @@ def touch(filename: str) -> str:
 def is_git_dir(d: "PathLike") -> bool:
     """This is taken from the git setup.c:is_git_directory function.
 
-    :raises WorkTreeRepositoryUnsupported:
+    :raise WorkTreeRepositoryUnsupported:
         If it sees a worktree directory. It's quite hacky to do that here, but at least
         clearly indicates that we don't support it. There is the unlikely danger to
         throw if we see directories which just look like a worktree dir, but are none.