[ci skip] Fixed typo. (#6151)

lib/associations/belongs-to.js

@@ -203,7 +203,7 @@ class BelongsTo extends Association {
     return Target.findOne(options);
     return Target.findOne(options);
   }
   }
 
 
-  // Add setAssociaton method to the prototype of the model instance
+  // Add setAssociation method to the prototype of the model instance
   injectSetter(instancePrototype) {
   injectSetter(instancePrototype) {
     const association = this;
     const association = this;