Fix outdated gem count in README.

Author: nirvdrum

README.md

@@ -24,7 +24,7 @@ Add `bootstrap` to your Gemfile:
 gem 'bootstrap', '~> 5.3.3'
 ```
 
-This gem requires a Sass engine, so make sure you have **one** of these two gems in your Gemfile:
+This gem requires a Sass engine, so make sure you have **one** of these gems in your Gemfile:
 - [`dartsass-sprockets`](https://github.com/tablecheck/dartsass-sprockets): Dart Sass engine, recommended but only works for Ruby 2.6+ and Rails 5+
 - [`dartsass-rails`](https://github.com/rails/dartsass-rails): Dart Sass engine, recommended for Rails projects that use Propshaft
 - [`cssbundling-rails`](https://github.com/rails/cssbundling-rails): External Sass engine