[2.1.x] Fixed typo in docs/internals/contributing/writing-code/coding-style.txt.

Backport of 5a2dd5ec from master

docs/internals/contributing/writing-code/coding-style.txt

@@ -36,13 +36,13 @@ Python style
 * Use four space hanging indentation rather than vertical alignment::
 
     raise AttributeError(
-        'Here is a multine error message '
+        'Here is a multiline error message '
         'shortened for clarity.'
     )
 
   Instead of::
 
-      raise AttributeError('Here is a multine error message '
+      raise AttributeError('Here is a multiline error message '
                            'shortened for clarity.')
 
   This makes better use of space and avoids having to realign strings if the