Replace Starter POM to Starter in the documentation

"Starter POM" is a confusing term as it implies the starter may be a
POM while it's actually a jar artifact like any other dependency. To
reduce the confusion (especially in the way such starter should be
declared in the build), the term Starter POM has been renamed to Starter.

Closes gh-5966

spring-boot-starters/README.adoc

@@ -1,4 +1,4 @@
-= Starter POMs
+= Starters
 
 Spring Boot Starters are a set of convenient dependency descriptors that you can include
 in your application. You get a one-stop-shop for all the Spring and related technology