Small typo.

[Brian Gough]

Alan Aspuru-Guzik writes:
 >  Hi fellow GSL'ers, I found out that there is a small typo in the
 > documentation for the sorting functions: There is no function
 > gsl_permutation_invert in any *.h file, and the
 > gsl_permutation_inverse function matches the previous prototype,
 > and seems to do what 'invert' was supposed to do in the example.

Thanks for the bug report... now fixed.

Brian