doc/go1.14: fix math/big.(*Int).GCD notes
    
    GCD is a method, not a function, so the link was broken.
    
    Change-Id: Icbb09d39959e7c71a48987b15bb82febe12c3a19
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217303
    Reviewed-by: Ian Lance Taylor <iant@golang.org>