[3.6] Make json.dumps() example to be PEP-8 compliant. (GH-3472) (GH-3473)

(cherry picked from commit a7fbad9)

Author: miss-islington

Doc/library/json.rst

@@ -43,7 +43,7 @@ Encoding basic Python object hierarchies::
 Compact encoding::
 
     >>> import json
-    >>> json.dumps([1,2,3,{'4': 5, '6': 7}], separators=(',', ':'))
+    >>> json.dumps([1, 2, 3, {'4': 5, '6': 7}], separators=(',', ':'))
     '[1,2,3,{"4":5,"6":7}]'
 
 Pretty printing::