runtime: fix typo, func comments should start with function name

Change-Id: I289af4884583537639800e37928c22814d38cba9
Reviewed-on: https://go-review.googlesource.com/98115Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

src/runtime/mbitmap.go

@@ -170,7 +170,7 @@ func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
 	return markBits{bytep, mask, allocBitIndex}
 }
 
-// refillaCache takes 8 bytes s.allocBits starting at whichByte
+// refillAllocCache takes 8 bytes s.allocBits starting at whichByte
 // and negates them so that ctz (count trailing zeros) instructions
 // can be used. It then places these 8 bytes into the cached 64 bit
 // s.allocCache.