doc(_call_process): correct example

Author: elovelan

git/cmd.py

@@ -1595,7 +1595,7 @@ def _call_process(
 
         turns into::
 
-            git rev-list --max-count=10 --header=master
+            git rev-list --max-count=10 --header master
 
         :return:
             Same as :meth:`execute`. If no args are given, used :meth:`execute`'s