Stray backslashes in html docs

[Trevor Spiteri]

In the html documentation, I found a few backslashes that shouldn't be
there:

https://gmplib.org/manual/FFT-Multiplication.html#FFT-Multiplication

mp\_bits\_per\_limb

https://gmplib.org/manual/Low_002dlevel-Functions.html#index-mpn_005fsec_005fpowm

GMP\_NUMB\_BITS

https://gmplib.org/manual/Low_002dlevel-Functions.html#index-mpn_005fsec_005finvert

\log (twice)

Regards.

Trevor Spiteri