runtime: fix typo in comment

Change-Id: I85f518e36c18f4f0eda8b167750b43cd8c48ecff
Reviewed-on: https://go-review.googlesource.com/c/go/+/622675


Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/runtime/mheap.go

@@ -431,12 +431,12 @@ type mspan struct {
 	// indicating a free object. freeindex is then adjusted so that subsequent scans begin
 	// just past the newly discovered free object.
 	//
-	// If freeindex == nelem, this span has no free objects.
+	// If freeindex == nelems, this span has no free objects.
 	//
 	// allocBits is a bitmap of objects in this span.
 	// If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0
 	// then object n is free;
-	// otherwise, object n is allocated. Bits starting at nelem are
+	// otherwise, object n is allocated. Bits starting at nelems are
 	// undefined and should never be referenced.
 	//
 	// Object n starts at address n*elemsize + (start << pageShift).