Use " quotes for $PYENV_ROOT in [[ -d ]] check

If a user sets `$PYENV_ROOT` to a path with ` ` (space) inside, the variable expansion might be not correct.

It is better to use explicit quoting.

libexec/pyenv-init

@@ -165,7 +165,7 @@ function help_() {
       fi
       echo
       echo 'export PYENV_ROOT="$HOME/.pyenv"'
-      echo '[[ -d $PYENV_ROOT/bin ]] && export PATH="$PYENV_ROOT/bin:$PATH"'
+      echo '[[ -d "$PYENV_ROOT/bin" ]] && export PATH="$PYENV_ROOT/bin:$PATH"'
       echo 'eval "$(pyenv init -)"'
       ;;
     esac