README: Fix typo (missing space) on Rails.application.config.dartsass.build_options

[navidemad]

In the README.md:
Rails.application.config.dartsass.build_options << "--no-charset" << "--quiet-deps"

Produces with a fresh app:

02:56:29 css.1  | Could not find an option named "no-source-map--no-charset--quiet-deps".
02:56:29 css.1  |
02:56:29 css.1  | Usage: sass <input.scss> [output.css]
02:56:29 css.1  |        sass <input.scss>:<output.css> <input/>:<output/> <dir/>

By default Rails.application.config.dartsass.build_options is:
--style=compressed --no-source-map

We have to add space before the first of each dashes.
Rails.application.config.dartsass.build_options << " --no-charset" << " --quiet-deps"

[ntkme]

This README change is intentional for version >= 0.5.1, which has been released.