cmd/asm/internal/lex: format error correctly

Error doesn't take a format string and appends its own newline. Phrase
the error like the other ones.

Change-Id: Ic3af857e5d4890207c74a6eb59a0d1067b503e1b
Reviewed-on: https://go-review.googlesource.com/16420Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/asm/internal/lex/input.go

@@ -261,7 +261,7 @@ func (in *Input) macroDefinition(name string) ([]string, []Token) {
 	// Scan to newline. Backslashes escape newlines.
 	for tok != '\n' {
 		if tok == scanner.EOF {
-			in.Error("missing newline in macro definition for %q\n", name)
+			in.Error("missing newline in definition for macro:", name)
 		}
 		if tok == '\\' {
 			tok = in.Stack.Next()