[docs] Fix unexpected indentation in HowToCrossCompileLLVM.rst

Seems like my sphynx version is different than the one in the bot, as it
accepted everything locally. I think this is the right fix...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269062 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed