cmd: fix kwarg formatting in docstring example

Update the example to accurately reflect the output of `transform_kwarg`.
When a key is longer than one letter and its value is a non-empty,
non-boolean type, it is transformed into the `--key=value` format,
rather than missing the double dashes or using spaces.

Signed-off-by: Uwe Schwaeke <uwe.schwaeke@clyso.com>

git/cmd.py

@@ -1572,7 +1572,7 @@ 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