Fixed incorrect type declaration. (#707)

ellens-alien-game: fix typo

exercises/concept/ellens-alien-game/.docs/introduction.md

@@ -30,7 +30,7 @@ Notice the `;` after the definition:
 ```cpp
 class Wizard {
   public:               // from here on all members are publicly accessible
-    inc cast_spell() {  // defines the public member function cast_spell
+    int cast_spell() {  // defines the public member function cast_spell
       return damage;
     }
     std::string name{}; // defines the public member variable `name`