osutil: reformat triple-quoted string so black doesn't confuse itself

Differential Revision: https://phab.mercurial-scm.org/D9429

mercurial/pure/osutil.py

@@ -293,7 +293,8 b' else:'
             '''mimics the read-only attributes of Python file objects
             by raising 'TypeError: readonly attribute' if someone tries:
               f = posixfile('foo.txt')
-              f.name = 'bla'  '''
+              f.name = 'bla'
+            '''
             return self._file.__setattr__(name, value)
 
         def __enter__(self):