Update README.md

Author: aviggiano

README.md

@@ -18,7 +18,7 @@ Pull requests are welcome.
 - redis (server needed for integration tests)
 - hiredis (redis client library needed for performance tests)
 
-##### Known issues
+##### Known limitations
 
 - This library only works with [32-bit integeres](https://github.com/RoaringBitmap/CRoaring/issues/1) (e.g. counting numbers up to 4294967296)