fix typo and add attribute group.

4e6e8af5e0 was not work well on my vagrant.
I found typo and (maybe) attribute "group" is missing.


I think, need to update README.md. (But I dont have any confidence)
```
plenv_install "5.16.3" do
  user   "kentaro"
...
```
to
```
plenv_install "5.16.3" do
  user   "kentaro"
  group  "kentaro"
...
```
(also plenv_cpanm, _global)