Update pub_sub.md (#420)

Fixed typo

_docs/tutorials/programming_rcl_rclc/pub_sub/pub_sub.md

@@ -22,7 +22,7 @@ Ready to use code related to this concepts can be found in [`micro-ROS-demos/rcl
 
 ### Initialization
 
-Starting from a code where RCL is initialized and a micro-ROS node is created, there are tree ways to initialize a publisher depending on the desired quality-of-service configuration:
+Starting from a code where RCL is initialized and a micro-ROS node is created, there are three ways to initialize a publisher depending on the desired quality-of-service configuration:
 
 - Reliable (default):
   ```c
@@ -249,4 +249,4 @@ rcl_publisher_fini(&publisher, &node);
 rcl_subscription_fini(&subscriber, &node);
 ```
 
-This will delete any automatically created infrastructure on the agent (if possible) and deallocate used memory on the client side.
+This will delete any automatically created infrastructure on the agent (if possible) and deallocate used memory on the client side.