Fixing verb in permdesc_readFrameBuffer English.

There were too many verbs in this sentence. Ideally this patch should
also be applied to some of the other languages. German seems OK but
French not - and that's as far as my linguistic abilities go. I
haven't fixed the French since I'm only 99% sure I wouldn't mess it up!

I chose "read" not "use" because this permission does not allow
writing to the frame buffer; it currently only controls the
screenshotting functionality inside SurfaceFlinger.