Fix a misspelling in the documentation.

Hi,

this change corrects a misspelling in the documentation.

I hereby disclaim any explicit or implicit copyright ownership of this change
and place it under your choice of the public domain / CC-Zero / the MIT X11
licence / the LGPL v2-or-above / the GPL v2-or-above / any other licence
of your choice.

Nevertheless, I would appreciate if you credit me (Shlomi Fish, whose
home page is http://www.shlomifish.org/ ).