runtime: fix incorrect comment about bitvector layout

Commit 303b69fe packed bitvectors more tightly, but missed a comment
describing their old layout. Update that comment.

Change-Id: I095ccb01f245197054252545f37b40605a550dec
Reviewed-on: https://go-review.googlesource.com/33718Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/runtime/symtab.go

@@ -666,7 +666,7 @@ func readvarint(p []byte) (newp []byte, val uint32) {
 type stackmap struct {
 	n        int32   // number of bitmaps
 	nbit     int32   // number of bits in each bitmap
-	bytedata [1]byte // bitmaps, each starting on a 32-bit boundary
+	bytedata [1]byte // bitmaps, each starting on a byte boundary
 }
 
 //go:nowritebarrier