Correct FreeBSD pkg instructions

From pkg(8):

     add     Install a package from either a local source or a remote one.

             When installing from remote source you need to specify the proto-
             col to use when fetching the package.

             Currently supported protocols are FTP, HTTP and HTTPS.

...

     install
             Install a package from a remote package repository.  If a package
             is found in more than one remote repository, then installation
             happens from the first one.  Downloading a package is tried from
             each package repository in turn, until the package is success-
             fully fetched.

i.e., add is to install a package with an explicit (local or remote) path, while install uses the repository configuration to install by name