git_config: fix _SaveJson typo

Change-Id: I35ca2b3733e6d1508669f9a6690c6645c582912e
diff --git a/git_config.py b/git_config.py
index c4c31e1..8ded7c2 100644
--- a/git_config.py
+++ b/git_config.py
@@ -280,7 +280,7 @@
       finally:
         fd.close()
     except (IOError, TypeError):
-      if os.path.exists(self.json):
+      if os.path.exists(self._json):
         os.remove(self._json)
 
   def _ReadGit(self):