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>
1 file changed