regexp: small correction to test comment
regexp: small correction to test comment

s/Backtrace/Backtrack/

Change-Id: I062aab18f23f2bc2110cf7210c2e7264747e02cf
Reviewed-on: https://go-review.googlesource.com/11091
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

matloob/regexp,matloob/regexp,matloob/regexp