DEV: Correct typos and spelling mistakes (#12812)

Over the years we accrued many spelling mistakes in the code base. 

This PR attempts to fix spelling mistakes and typos in all areas of the code that are extremely safe to change 

- comments
- test descriptions
- other low risk areas

spec/support/shared_examples_for_stats_cacheable.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-shared_examples_for 'stats cachable' do
+shared_examples_for 'stats cacheable' do
   describe 'fetch_cached_stats' do
     after do
       Discourse.redis.del(described_class.stats_cache_key)