[WebProfilerBundle] Fix regexp

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12122
| License       | MIT
| Doc PR        | 

I fixed the regexp for don't see `/app_dev.php/_wdt`.
why 3 backslashes ?