Fix formatting on cache-url README
Fix formatting on cache-url README

- Fixes the markdown markup to format and syntax highlight the code.

Change-Id: I7fbea2fa217756e03b1e386a4dad6fa31a8a67ce

google/amp-toolbox,ampproject/amp-toolbox,ampproject/amp-toolbox,google/amp-toolbox,google/amp-toolbox,ampproject/amp-toolbox,ampproject/amp-toolbox