Add primary group in the first user module doc example

##### Issue Type:

“Docs Pull Request”.

##### Ansible Version:

ansible 1.5.3

##### Environment:

Debian Jessie - Virtualenv Python 2.7

##### Summary:

The primary group is missing in this example

    # Add the user 'johnd' with a specific uid and a primary group of 'admin'
    - user: name=johnd comment="John Doe" uid=1040

##### Steps To Reproduce:

nothing

##### Expected Results:

    # Add the user 'johnd' with a specific uid and a primary group of 'admin'
    - user: name=johnd comment="John Doe" uid=1040 group=admin

##### Actual Results:

    # Add the user 'johnd' with a specific uid and a primary group of 'admin'
    - user: name=johnd comment="John Doe" uid=1040