Documentation/llvm: turn make command for ccache into code block

The command provided to use ccache with clang is not a literal code
block. Once built, the documentation displays the '' symbols as a "
character, which is wrong, and the command can not be applied as
provided.

Turn the command into a literal code block.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>