doc: Fix incorrect enum name in QRawFont::alphaMapForGlyph docs

doc: Fix incorrect enum name in QRawFont::alphaMapForGlyph docs

There's no QImage::Format_A8 format. This is a term used in the
corresponding QFontEngineGlyphCache enum.

Reviewed-by: Jiang Jiang
(cherry picked from commit e453fce2731069dce993a94b0c2291307cc7806c)

Change-Id: I68a190abdf4446d05231af89ac861380ac79b188
Reviewed-on: http://codereview.qt-project.org/4773
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>