runtime: update malloc comment for MSpan.needzero
runtime: update malloc comment for MSpan.needzero

Missed this suggestion in CL 57680046.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/63390043

tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme