readme: fix typo

Author: aak74

README.md

@@ -4,7 +4,7 @@ This package can turn Tarantool into a persistent memcache replacement,
 but is powerful enough so that  your own expiration strategy can be defined.
 
 You define two functions: one takes a tuple as an input and returns
-true in case it's expirted and false otherwise. The other takes the
+true in case it's expired and false otherwise. The other takes the
 tuple and performs the expiry itself: either deletes it (memcache), or
 does something smarter, like put a smaller representation of the data
 being deleted into some other space.