cmd/compile/internal/ssa: fix a typo

Change-Id: Ie3a8c54fe5e1b94f506cc0e6f650aab441d28a75
Reviewed-on: https://go-review.googlesource.com/137115Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/prove.go

@@ -625,7 +625,7 @@ var (
 	// For example:
 	//      OpLess8:   {signed, lt},
 	//	v1 = (OpLess8 v2 v3).
-	// If v1 branch is taken than we learn that the rangeMaks
+	// If v1 branch is taken then we learn that the rangeMask
 	// can be at most lt.
 	domainRelationTable = map[Op]struct {
 		d domain