fix typo

Author: xuyuan

introduction/set_joint_commands.py

@@ -5,7 +5,7 @@
     1. set stiffness of LShoulderPitch to 0
     2. set speed of HeadYaw to 0.1
 
-* Hint: The commands are store in action (class Action in spark_agent.py)
+* Hint: The commands are stored in action (class Action in spark_agent.py)
 
 '''