Spring Fix typo in the Simple Broker section of the reference documentation

Fixed a small typo.

Link to the relevant webpage: https://docs.spring.io/spring-framework/reference/web/websocket/stomp/handle-simple-broker.html

Comment From: pivotal-cla

@hlmg Please sign the Contributor License Agreement!

Click here to manually synchronize the status of this Pull Request.

See the FAQ for frequently asked questions.

Comment From: pivotal-cla

@hlmg Thank you for signing the Contributor License Agreement!

Comment From: snicoll

Good catch, @hlmg. And thank you for making your first contribution to Spring Framework.