MONGOID-5476 update 8.0 installation version (#5463)

Author: neilshweky

docs/installation.txt

@@ -27,7 +27,7 @@ To install the gem with bundler, include the following in your ``Gemfile``:
 
 .. code-block:: ruby
 
-   gem 'mongoid', '~> 7.3.0'
+   gem 'mongoid', '~> 8.0.2'
 
 Using Mongoid with a New Rails Application
 ==========================================