Update services.md (#393)

Spelling Correction

Author: nlinthacum

_docs/tutorials/programming_rcl_rclc/service/services.md

@@ -22,7 +22,7 @@ Ready to use code related to this concepts can be found in [`micro-ROS-demos/rcl
 ## Service server
 
 ### Initialization
-Starting from a code where RCL is initialized and a micro-ROS node is created, there are tree ways to initialize a service server:
+Starting from a code where RCL is initialized and a micro-ROS node is created, there are three ways to initialize a service server:
 
 - Reliable (default):