Update mercure.rst

[jannes-io]

Ensure people install the correct bundle

When you follow the docs now:

composer require mercure
Could not find package mercure.
Pick one of these or leave empty to abort:
  [0] anik/mercure
  [1] symfony/mercure
  [2] freddie/mercure-x
  [3] bizley/yii2-mercure
  [4] symfony/ux-turbo
 > 1

This can also be considered a security issue, if someone publishes a mercure package that contains malicious code, this could lead to major problems...

[jannes-io]

Nevermind.

When you require mercure in a symfony application it installs correctly. When you require it in a symfony bundle, you must require symfony/mercure-bundle.