cmd/compile: fix comment to reference runtime.countrunes

Updates #24923

Change-Id: Ie5a1b54b023381b58df618080f3d742a50d46d8b
Reviewed-on: https://go-review.googlesource.com/115836Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/walk.go

@@ -4078,7 +4078,7 @@ func canMergeLoads() bool {
 }
 
 // isRuneCount reports whether n is of the form len([]rune(string)).
-// These are optimized into a call to runtime.runecount.
+// These are optimized into a call to runtime.countrunes.
 func isRuneCount(n *Node) bool {
 	return Debug['N'] == 0 && !instrumenting && n.Op == OLEN && n.Left.Op == OSTRARRAYRUNE
 }