benchmark/parse: fix cosmetic defect in error str.
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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

dmitris/tools,jdhenke/tools,syreclabs/go-tools,matryer/tools,runcom/tools,cmars/tools,sshetty10/tools,jdhenke/tools,josharian/tools,josharian/tools,syreclabs/go-tools,golang/tools,springning/tools,cw2018/tools,jdhenke/tools,cw2018/tools,argandas/tools,argandas/tools,magiccao/tools,cw2018/tools,nicot/tools,springning/tools,runcom/tools,dmitris/tools,syreclabs/go-tools,matryer/tools,matryer/tools,maddyonline/tools,nicot/tools,cmars/tools,magiccao/tools,maddyonline/tools,cmars/tools,syreclabs/go-tools,magiccao/tools,syreclabs/go-tools,glasser/golang-tools,glasser/golang-tools,argandas/tools,sshetty10/tools,magiccao/tools,maddyonline/tools,golang/tools,nicot/tools,golang/tools,springning/tools,runcom/tools,josharian/tools,springning/tools,dmitris/tools,golang/tools,argandas/tools,josharian/tools,glasser/golang-tools,syreclabs/go-tools,nicot/tools,matryer/tools,cw2018/tools,golang/tools,maddyonline/tools,runcom/tools,jdhenke/tools,dmitris/tools,glasser/golang-tools,sshetty10/tools,cmars/tools,sshetty10/tools