fix deprecated instructions in readme

[adrum]

Description

Before

php ./craft install/plugin mailgun

                                                 
    The install/plugin command is deprecated.    
        Running plugin/install instead...        
                                                 

*** installing mailgun
*** installed mailgun successfully (time: 0.014s)

After

php ./craft plugin/install mailgun

*** installing mailgun
*** installed mailgun successfully (time: 0.000s)

[adrum]

Is anything else needed from me? I'm going through and cleaning out old PRs