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>

Documentation/kbuild/llvm.rst

@@ -126,7 +126,7 @@ Ccache
 
 ``ccache`` can be used with ``clang`` to improve subsequent builds, (though
 KBUILD_BUILD_TIMESTAMP_ should be set to a deterministic value between builds
-in order to avoid 100% cache misses, see Reproducible_builds_ for more info):
+in order to avoid 100% cache misses, see Reproducible_builds_ for more info)::
 
 	KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang"