Fix double escaped backtrace links in TextMateFormatter.

This was introduced in https://github.com/rspec/rspec-core/commit/b295e165658c0cc2f72a14d8c666a552d4b9b489

The regression went unnoticed because the spec `produces HTML identical to the one we designed manually` was flawed: It did some sanity checking for each backtrace link, but it failed to fail :smirk: if there were no links at all. Which exactly is what happened when the double escape was introduced: the backtrace did no longer contain any (unescaped) links.