_config.yml: plugins config option deprecated

[tracernz]

When building the site:

Configuration file: /home/mike/dev/d-ronin.github.io/_config.yml
       Deprecation: The 'plugins' configuration option has been renamed to 'plugins_dir'. Please update your config file accordingly.

[mlyle]

Comes from upstream (minimal-mistakes). They have it still because the rename is brand new (and the second rename in Jekyll in a few monhs of this parameter).

Probably should not touch this, because it'll just cause conflicts whenever m-m decides it's appropriate to rename.