Mercurial plugin: revise README
Mercurial plugin: revise README

Closes #4616
Changes the `plugins=(... mercurial ...)` to be closer to correct zsh syntax, removing commas and extra spaces.
Revises discussion on configuring themes to avoid sounding like users should modify the theme definition file in the main OMZ folder. Prioritizes the suggestion of using a theme which already supports hg_prompt_info.

FrankSansC/oh-my-zsh,FrankSansC/oh-my-zsh,FrankSansC/oh-my-zsh,FrankSansC/oh-my-zsh