[libm] Correct comments in s_cbrt[l].c

Damian McGuckin <damianm at esi dot com dot au> noted that the accuracy
claims in the code for cbrt(3) and cbrtl(3) were incorrect. Fix the
comments to more accurately describe the accuracies.

PR: 265603
MFC after: 3 days

(cherry picked from commit 369ea0520a3061c07400d7cd32172efb6af39815)