SF bug #1567976 : fix typo(backport from r52048)
SF bug #1567976 : fix typo(backport from r52048)

Since this paragraph is about exception's message attribute(2.5 feature),
I will not backport to 2.4 branch.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator