Fix YAML formatting in pr2_arm_controllers.yaml (ros ticket #5021)

[ahendrix]

The YAML formatting in the existing file makes a user have to change two values to change gain values.

Attached is a YAML file that won't do that.

trac data:

• Owner: watts
• Reporter: watts
• Reported at: Tue Feb 15 15:23:57 2011
• URL: https://code.ros.org/trac/wg-ros-pkg/ticket/5021

[ahendrix]

[sglaser] LGTM. Go ahead and check that in.

[ahendrix]

[watts] https://kforge.ros.org/pr2robot/hg/rev/75844cdee405