[Notifier] Fix wrong package name for the RocketChat service

[tambait]

Package name "symfony/rocketchat-notifier" should be "symfony/rocket-chat-notifier"

Installation of the package using current package name returns InvalidArgumentException

[InvalidArgumentException]
Could not find a matching version of package symfony/rocketchat-notifier.
Check the package spelling, your version constraint and that the package is available 
in a stability which matches your minimum-stability (stable).

[OskarStark]

Good catch, thanks Ivan.