readme: Fix typo (#174)

Author: rmandvikar

README.md

@@ -65,7 +65,7 @@ For .net framework 4.0 use LazyCache 0.6
 
 - Simple API with familiar sliding or absolute expiration
 - Guaranteed single evaluation of your factory delegate whose results you want to cache
-- Strongly typed generics based API. No need to cast your cached objects every time you retieve them
+- Strongly typed generics based API. No need to cast your cached objects every time you retrieve them
 - Stops you inadvertently caching an exception by removing Lazys that evaluate to an exception
 - Thread safe, concurrency ready
 - Async compatible - lazy single evaluation of async delegates using `GetOrAddAsync()`