Update polymorphism.md (#774)

Fixed error

pages/docs/polymorphism.md

@@ -40,7 +40,7 @@ You can specify polymorphism properties separately using the following GORM tags
 type Dog struct {
   ID   int
   Name string
-  Toys Toy `gorm:"polymorphicType:Kind;polymorphicId:OwnerID;polymorphicValue:master"`
+  Toys []Toy `gorm:"polymorphicType:Kind;polymorphicId:OwnerID;polymorphicValue:master"`
 }
 
 type Toy struct {
@@ -55,4 +55,4 @@ db.Create(&Dog{Name: "dog1", Toys: []Toy{{Name: "toy1"}, {Name: "toy2"}}})
 // INSERT INTO `toys` (`name`,`owner_id`,`kind`) VALUES ("toy1",1,"master"), ("toy2",1,"master")
 ```
 
-In these examples, we've used a has-many relationship, but the same principles apply to has-one relationships.
+In these examples, we've used a has-many relationship, but the same principles apply to has-one relationships.