fix: correct grammar (#10981)

lib/model.js

@@ -329,7 +329,7 @@ class Model {
   static _transformStringAssociation(include, self) {
     if (self && typeof include === 'string') {
       if (!self.associations.hasOwnProperty(include)) {
-        throw new Error(`Association with alias "${include}" does not exists`);
+        throw new Error(`Association with alias "${include}" does not exist`);
       }
       return self.associations[include];
     }