Issue #26114: Remove a reference to 'Numerical Recipes'.

While no copyright violation occurred, the license which
'Numerical Recipes' operates under is not amenable to Python,
so to prevent confusion it's easier to simply remove its mention.

Modules/mathmodule.c

@@ -400,9 +400,8 @@ m_lgamma(double x)
    Implementations of the error function erf(x) and the complementary error
    function erfc(x).
 
-   Method: following 'Numerical Recipes' by Flannery, Press et. al. (2nd ed.,
-   Cambridge University Press), we use a series approximation for erf for
-   small x, and a continued fraction approximation for erfc(x) for larger x;
+   Method: we use a series approximation for erf for small x, and a continued
+   fraction approximation for erfc(x) for larger x;
    combined with the relations erf(-x) = -erf(x) and erfc(x) = 1.0 - erf(x),
    this gives us erf(x) and erfc(x) for all x.