Reduce Sentry sample size to 10%

We noticed that the Sentry interface got very sluggish and the instance was running at 80-90% CPU. This is due to the increased amount of events we were sending after @ironsidevsquincy merged #5698.

I have since doubled the ASG instance size to 2, and now the instances are running at 40%, but until we can discuss with someone the health of the RDS database instance I want to reduce the load.

This PR simply decreases the sample size back down to 10% in the interim.

/cc @ironsidevsquincy @rich-nguyen