Minor documentation refinements (backported from 5.1.x)

Author: jhoeller

spring-messaging/src/main/java/org/springframework/messaging/simp/package-info.java

@@ -1,4 +1,4 @@
 /**
- * Generic support for SImple Messaging Protocols including protocols such as STOMP.
+ * Generic support for Simple Messaging Protocols including protocols such as STOMP.
  */
 package org.springframework.messaging.simp;