[git-clang-format] Change run line from python to python3

Several systems no longer ship `python`.

Differential Revision: https://reviews.llvm.org/D124280

NOKEYCHECK=True
GitOrigin-RevId: c94a02e0e223bcce3b2c88741fae35d31a2d4f1d
diff --git a/git-clang-format b/git-clang-format
index 86f4497..96f415e 100755
--- a/git-clang-format
+++ b/git-clang-format
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 #===- git-clang-format - ClangFormat Git Integration ---------*- python -*--===#
 #