Added override keyword near IntRange::describe() method

docs/matchers.md

@@ -100,7 +100,7 @@ public:
     // include any provided data (the begin/ end in this case) and
     // be written as if it were stating a fact (in the output it will be
     // preceded by the value under test).
-    virtual std::string describe() const {
+    virtual std::string describe() const override {
         std::ostringstream ss;
         ss << "is between " << m_begin << " and " << m_end;
         return ss.str();