cmd/go: fix error message

Change-Id: I440435927fc9f417d81ed4e8656a73787bf0968a
Reviewed-on: https://go-review.googlesource.com/8801Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/testflag.go

@@ -170,7 +170,7 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 			case "set", "count", "atomic":
 				testCoverMode = value
 			default:
-				fatalf("invalid flag argument for -cover: %q", value)
+				fatalf("invalid flag argument for -covermode: %q", value)
 			}
 			testCover = true
 		case "outputdir":