Merge pull request matplotlib#10467 from ImportanceOfBeingErnest/patch-8

update example docstring of afm.py

Author: anntzer

lib/matplotlib/afm.py

@@ -19,7 +19,7 @@
     ...                         'fonts', 'afm', 'ptmr8a.afm')
     >>>
     >>> from matplotlib.afm import AFM
-    >>> with open(afm_fname) as fh:
+    >>> with open(afm_fname, 'rb') as fh:
     ...     afm = AFM(fh)
     >>> afm.string_width_height('What the heck?')
     (6220.0, 694)