Fix typo in string/bits/string2.h.

The comment above the bzero() macro in this file appears to have been
copied verbatim from the comment above the memset() prototype in
string.h proper.  bzero() has no 'c' argument and can only set memory
contents to 0.  (The comment above the prototype of bzero() in
string.h proper does not make the same mistake.)

	* string/bits/string2.h: Fix typo in comment.