Update the wrong variable used in rb20 function. Fixes #1762

Signed-off-by: Joni Chandra <joniliciously@gmail.com>

plugins/rvm/rvm.plugin.zsh

@@ -31,7 +31,7 @@ compdef _rb19 rb19
 
 function rb20 {
 	if [ -z "$1" ]; then
-		rvm use "$ruby"
+		rvm use "$ruby20"
 	else
 		rvm use "$ruby20@$1"
 	fi