Fix plugin add command in README.md

[Koenkk]

Seems the syntax has changed in recent asdf versions:

➜  asdf plugin-add dotnet-core https://github.com/emersonsoares/asdf-dotnet-core.git
invalid command provided: plugin-add

version: v0.18.0 (revision 2114f1e)

MANAGE PLUGINS
asdf plugin add <name> [<git-url>]      Add a plugin from the plugin repo OR,
                                        add a Git repo as a plugin by
                                        specifying the name and repo url
asdf plugin list [--urls] [--refs]      List installed plugins. Optionally show
                                        git urls and git-ref
asdf plugin list all                    List plugins registered on asdf-plugins
                                        repository with URLs
asdf plugin remove <name>               Remove plugin and package versions
asdf plugin update <name> [<git-ref>]   Update a plugin to latest commit on
                                        default branch or a particular git-ref
asdf plugin update --all                Update all plugins to latest commit on
                                        default branch