Click on the small icon with two curly braces.
Notice that the pretty-print icon has become blue to indicate that it is active. In the above picture, I am viewing the JS file in the Network tab. In the network tab, as long as the pretty-print button is in an active state, the files will be auto-formatted.
Things work slightly differently in the Sources tab. The pretty-print button is available here as well. But when you click on it, a new tab with the formatted code will open and become active. The old non-formatted tab will remain.