rbenv: fix current_gemset

Fixes #8925
Co-authored-by: Josh Goodall <inopinatus@inopinatus.org>

plugins/rbenv/rbenv.plugin.zsh

@@ -34,7 +34,7 @@ if [[ $FOUND_RBENV -eq 1 ]]; then
     }
 
     function current_gemset() {
-        echo "$(rbenv gemset active 2>/dev/null | sed -e ":a" -e '$ s/\n/+/gp;N;b a' | head -n1)"
+        echo "$(rbenv gemset active 2>/dev/null)" | tr ' ' '+'
     }
 
     function gems() {