Readme: fix code syntax in src block

[j-steinbach]

Added a missing -- before a comment. This has been annoying me for a long time.

(The string/list in line 400 should probably also be turned into a string or list, but that would break the indent of the comment cascade..)

[wbthomason]

Thanks! This is also fixed in #961 - as I commented there, do you think just removing the newline would be acceptable? It seems like a simpler fix to me if it doesn't cause unpleasant scrolling on the rendered README.

[j-steinbach]

I used the style that was already in the document (line 399), but choose whichever you prefer :)

This PR:

                               -- with one of these module names, the plugin will be loaded.
  module_pattern = string/list -- Specifies Lua pattern of Lua module names for require. When
                               -- requiring a string which matches one of these patterns, the plugin will be loaded.
}

No newline:

                               -- with one of these module names, the plugin will be loaded.
  module_pattern = string/list -- Specifies Lua pattern of Lua module names for require. When requiring a string which matches one of these patterns, the plugin will be loaded.
}

Having to scroll to the side is a (minor) annoyance, though. And there is a reason why newspapers limit their paragraph width to a limited amount of characters, even though they have way more space available.