cmd/compile: fix a typo in assignment mismatch error

Fixes #30087

Change-Id: Ic6d80f8e6e1831886af8613420b1bd129a1b4850
Reviewed-on: https://go-review.googlesource.com/c/161577Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -3603,7 +3603,7 @@ func typecheckas2(n *Node) {
 mismatch:
 	switch r.Op {
 	default:
-		yyerror("assignment mismatch: %d variable but %d values", cl, cr)
+		yyerror("assignment mismatch: %d variables but %d values", cl, cr)
 	case OCALLFUNC, OCALLMETH, OCALLINTER:
 		yyerror("assignment mismatch: %d variables but %v returns %d values", cl, r.Left, cr)
 	}

test/fixedbugs/issue30087.go

+// errorcheck
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main() {
+	var a, b = 1    // ERROR "assignment mismatch: 2 variables but 1 values"
+	_ = 1, 2        // ERROR "assignment mismatch: 1 variables but 2 values"
+	c, d := 1       // ERROR "assignment mismatch: 2 variables but 1 values"
+	e, f := 1, 2, 3 // ERROR "assignment mismatch: 2 variables but 3 values"
+}