docs(datastore): replace deprecated func in docs (#8989)

Author: takanakahiko

datastore/doc.go

@@ -220,7 +220,7 @@ Example code:
 		}
 
 		var entities []MyEntity
-		q := datastore.NewQuery("Entity").Filter("A =", 12).Limit(1)
+		q := datastore.NewQuery("Entity").FilterField("A", "=", 12).Limit(1)
 		if _, err := dsClient.GetAll(ctx, q, &entities); err != nil {
 			// Handle error
 		}

datastore/example_test.go

@@ -271,7 +271,7 @@ func ExampleClient_Run() {
 	}
 	// List the posts published since yesterday.
 	yesterday := time.Now().Add(-24 * time.Hour)
-	q := datastore.NewQuery("Post").Filter("PublishedAt >", yesterday)
+	q := datastore.NewQuery("Post").FilterField("PublishedAt", ">", yesterday)
 	it := client.Run(ctx, q)
 	_ = it // TODO: iterate using Next.
 }