Update README.md for typo

Author: simonharris

README.md

@@ -7,7 +7,7 @@ PlUnit is a unit testing library for Prolog. Its goals are:
 2. provide significantly more helpful and user-friendly feedback on test fail
 3. full compatibility with PlUnit: you can still use  `assertion/1` if you wish
 
-Whilst this wasn't an explicit design goal initially, another benefit of `plunit_asssert` is:
+Whilst this wasn't an explicit design goal initially, another benefit of `plunit_assert` is:
 
 4. it can easily be leveraged by calling its test predicates directly within the Prolog REPL, which can be a trying experience with manual calls to `assertion/1`.