Remove uses of "--" as constant prefix

Code now uses the prefixForMethod(...)
added sample usage of prefix in a @ShellMethod

spring-shell2-samples/src/main/java/org/springframework/shell2/samples/standard/Commands.java

@@ -44,8 +44,13 @@ public class Commands {
 	}
 
 	@ShellMethod(help = "it's cool")
-	public void foo(String bar) {
-
+	public String foo(String bar) {
+		return bar;
+	}
+	
+	@ShellMethod(help = "it's cool with a prefix", prefix = "-")
+	public String fooPrefix(String bar) {
+		return bar;
 	}
 
 	@ShellMethod(help = "Shows support for boolean parameters, with arity=0")