Fix pretty_format_json to use int indent

The indent parameter for json should be integer and under Python2 is
will raise an error if not. So switch from str to int and mention
default value in help text.

pre_commit_hooks/pretty_format_json.py

@@ -33,24 +33,12 @@ def _autofix(filename, new_contents):
         f.write(new_contents)
 
 
-def parse_indent(s):
-    # type: (str) -> str
+def parse_num_to_int(s):
+    """Convert string numbers to int, leaving strings as is."""
     try:
-        int_indentation_spec = int(s)
+        return int(s)
     except ValueError:
-        if not s.strip():
-            return s
-        else:
-            raise ValueError(
-                'Non-whitespace JSON indentation delimiter supplied. ',
-            )
-    else:
-        if int_indentation_spec >= 0:
-            return int_indentation_spec * ' '
-        else:
-            raise ValueError(
-                'Negative integer supplied to construct JSON indentation delimiter. ',
-            )
+        return s
 
 
 def parse_topkeys(s):
@@ -68,9 +56,12 @@ def pretty_format_json(argv=None):
     )
     parser.add_argument(
         '--indent',
-        type=parse_indent,
-        default='  ',
-        help='String used as delimiter for one indentation level',
+        type=parse_num_to_int,
+        default='2',
+        help=(
+            'The number of indent spaces or a string to be used as delimiter'
+            ' for indentation level e.g. 4 or "\t" (Default: 2)'
+        ),
     )
     parser.add_argument(
         '--no-ensure-ascii',