Rewrite tosa-opt readme for better markdown rendering

Fixes some issues with markdown rendering and adjusts the installation
commands to make sure executables are on the users path and reduce
duplication of paths using `TRANSLATOR_BUILD` and `LLVM_BUILD`.

Change-Id: Ifdaffe8235e27ea6d9fbfb146afc897ecd4c1acf
Signed-off-by: Luke Hutton <luke.hutton@arm.com>