[core] Fix an incorrect assertion in the Score doc test

Negate with - instead of with !.

core/src/score.rs

@@ -23,7 +23,7 @@ impl Score {
     ///
     /// ```
     /// use chessfriend_core::score::Score;
-    /// assert_eq!(!Score::MIN, Score::MAX);
+    /// assert_eq!(-Score::MIN, Score::MAX);
     /// ```
     ///
     pub const MIN: Score = Score(Value::MIN + 1);