[docs] Amend confusing title

"Writing an LLVM Compiler Backend" can be misinterpreted as meaning
"backend" in the sense of "using LLVM as a backend for your compiler for
your new language". This new name is less ambiguous.

As a bonus, this brings the title in line with the file name.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185377 91177308-0d34-0410-b5e6-96231b3b80d8