docs: fix HeaderRegexp example (#3222)

From https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For

> Elements are comma-separated, with optional whitespace surrounding the commas.
> If a request goes through multiple proxies, the IP addresses of each successive proxy is listed.
> This means that, given well-behaved client and proxies, the rightmost IP address is the IP address
> of the most recent proxy and the leftmost IP address is the IP address of the originating client.

Fix example by removing trailing space.

Signed-off-by: Alexander Yastrebov <[email protected]>

docs/reference/predicates.md

@@ -309,7 +309,7 @@ Parameters:
 Examples:
 
 ```
-HeaderRegexp("X-Forwarded-For", "^192\.168\.0\.[0-2]?[0-9]?[0-9] ")
+HeaderRegexp("X-Forwarded-For", "^192\.168\.0\.[0-2]?[0-9]?[0-9]")
 HeaderRegexp("Accept", "application/(json|xml)")
 ```