Minor corrections/changes to the gmpbench.html documentation

[Will Galway]

I'm attaching a revised version for the html document that resides at
  http://gmplib.org/gmpbench.html
although perhaps just pointing out the changes makes more sense.  There
are two changes: "... work off the caches" in the first paragraph now
reads " ... works off the caches", and later on, more as a matter of
taste, I've changed "... added benchmarks of GMPbench 0.2 ..." to read
"... additional benchmarks of GMPbench 0.2 ...".

Do with this what you will.

-- Regards, Will Galway