Fix readme

README.md

@@ -61,8 +61,9 @@ struct Ping { id: i32 }
 
 impl Cacheable for Ping {
     fn cache_key(&self) -> Result<String, CacheError> {
-        Ok(format!("Ping::{}", self.id))
+        Ok(format!("{}::{}", self.cache_key_prefix(), self.id))
     }
+    fn cache_key_prefix(&self) -> String { "Ping".to_owned() }
 }
 ```
 Next step is to instantiate [Cache] actor with selected backend: