benchmark/parse: fix cosmetic defect in error str.

The "Benchmark" header parser's error string lacked a closing double
quotation mark.

Change-Id: I2361cc86866296503f2733aa84b3fd52cdf33c09
Reviewed-on: https://go-review.googlesource.com/3683
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

benchmark/parse/parse.go

@@ -46,7 +46,7 @@ func ParseLine(line string) (*Benchmark, error) {
 		return nil, fmt.Errorf("two fields required, have %d", len(fields))
 	}
 	if !strings.HasPrefix(fields[0], "Benchmark") {
-		return nil, fmt.Errorf(`first field does not start with "Benchmark`)
+		return nil, fmt.Errorf(`first field does not start with "Benchmark"`)
 	}
 	n, err := strconv.Atoi(fields[1])
 	if err != nil {