os: fix typo in TestExecutable comment
os: fix typo in TestExecutable comment

Change-Id: I2ad5d73deaee57ccbb2c75378dcc2884e43be3bb
Reviewed-on: https://go-review.googlesource.com/82899
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

Cofyc/go,golang/go,josharian/go,josharian/go,codestation/go,codestation/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,codestation/go,CAFxX/go,Cofyc/go,christopher-henderson/Go,Cofyc/go,nawawi/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,josharian/go,CAFxX/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go,Cofyc/go,josharian/go,golang/go,golang/go,akutz/go,akutz/go,codestation/go,cloudflare/go,Cofyc/go,akutz/go,josharian/go,akutz/go,codestation/go,nawawi/go,CAFxX/go,CAFxX/go,Cofyc/go,cloudflare/go,golang/go,codestation/go,Cofyc/go,josharian/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,codestation/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,codestation/go,nawawi/go,CAFxX/go,josharian/go,Cofyc/go