[clang-tools-extra] [clang-doc] Fix whitespace issues in Mustache basic test (PR #168491)

[Paul Kirth via cfe-commits]

ilovepi wrote:

Can you check if the output changes if we just ran the templates through an HTML formatter? I threw the HTML into just a random one from a google search to see (https://www.freeformatter.com/) and it did indent the partial the way you have it, and likely some more things too. If the output is not different or looks correct, then we should probably just format them all.

https://github.com/llvm/llvm-project/pull/168491