fix README typo

Author: sergioro9

README.md

@@ -48,7 +48,7 @@ Then, remove all the `*= require` and `*= require_tree` statements from the Sass
 Do not use `*= require` in Sass or your other stylesheets will not be able to access the Bootstrap mixins and variables.
 
 Bootstrap JavaScript can optionally use jQuery.
-If you're using Rails 5.1+, you can add add the `jquery-rails` gem to your Gemfile:
+If you're using Rails 5.1+, you can add the `jquery-rails` gem to your Gemfile:
 
 ```ruby
 gem 'jquery-rails'