A minor "typo" in the GMP documentation of mpn_mul_1

[Will Galway]

Edition 5.0.5 of the GMP manual, in the section on mpn_mul_1, states:
   Don't call this function if S2LIMB is a power of 2; use `mpn_lshift' 
with a count equal to the logarithm
   of S2LIMB instead, for optimal speed.

Of course, it would be more correct to write "base-2 logarithm" instead 
of "logarithm".

-- Regards, Will