updated the plugin according to the suggested changes

Co-authored-by: Koichi Murase <myoga.murase@gmail.com>

plugins/rbenv/rbenv.plugin.sh

@@ -1,6 +1,6 @@
 #! bash oh-my-bash.module
 
-if [ -d ~/.rbenv ]; then
+if [[ -d ~/.rbenv ]]; then
   export RBENV_ROOT=~/.rbenv
 	export PATH=$RBENV_ROOT/bin:$PATH
   eval "$(rbenv init - bash)"