slice.md, fixed gramatical error

If you want know more -> If you want to know more

en/src/compound-types/slice.md

@@ -85,7 +85,7 @@ fn main() {
     let mut s = String::from("hello world");
 
     // Here, &s is `&String` type, but `first_word` need a `&str` type.
-    // It works because `&String` can be implicitly converted to `&str. If you want know more, this is called `Deref coercion`. 
+    // It works because `&String` can be implicitly converted to `&str. If you want to know more, this is called `Deref coercion`. 
     let word = first_word(&s);
 
     s.clear(); // error!