Corrected typo and wording.

activesupport/lib/active_support/cache.rb

@@ -214,7 +214,7 @@ def self.instrument
       #   cache.write(key, value, :expire_in => 1.minute)  # Set a lower value for one entry
       #
       # Setting <tt>:race_condition_ttl</tt> is very useful in situations where a cache entry
-      # is used very frequently unver heavy load. If a cache expires and due to heavy load
+      # is used very frequently and is under heavy load. If a cache expires and due to heavy load
       # seven different processes will try to read data natively and then they all will try to
       # write to cache. To avoid that case the first process to find an expired cache entry will
       # bump the cache expiration time by the value set in <tt>:race_condition_ttl</tt>. Yes