Add deferred context cancellations to README examples (#441)

Author: Divjot Arora

README.md

@@ -87,7 +87,8 @@ Calling `Connect` does not block for server discovery. If you wish to know if a
 use the `Ping` method:
 
 ```go
-ctx, _ = context.WithTimeout(context.Background(), 2*time.Second)
+ctx, cancel = context.WithTimeout(context.Background(), 2*time.Second)
+defer cancel()
 err = client.Ping(ctx, readpref.Primary())
 ```
 
@@ -100,15 +101,17 @@ collection := client.Database("testing").Collection("numbers")
 The `Collection` instance can then be used to insert documents:
 
 ```go
-ctx, _ = context.WithTimeout(context.Background(), 5*time.Second)
+ctx, cancel = context.WithTimeout(context.Background(), 5*time.Second)
+defer cancel()
 res, err := collection.InsertOne(ctx, bson.M{"name": "pi", "value": 3.14159})
 id := res.InsertedID
 ```
 
 Several query methods return a cursor, which can be used like this:
 
 ```go
-ctx, _ = context.WithTimeout(context.Background(), 30*time.Second)
+ctx, cancel = context.WithTimeout(context.Background(), 30*time.Second)
+defer cancel()
 cur, err := collection.Find(ctx, bson.D{})
 if err != nil { log.Fatal(err) }
 defer cur.Close(ctx)
@@ -130,7 +133,8 @@ var result struct {
     Value float64
 }
 filter := bson.M{"name": "pi"}
-ctx, _ = context.WithTimeout(context.Background(), 5*time.Second)
+ctx, cancel = context.WithTimeout(context.Background(), 5*time.Second)
+defer cancel()
 err = collection.FindOne(ctx, filter).Decode(&result)
 if err != nil {
     log.Fatal(err)