Documentation/kbuild: modules.txt cleanup

A few modifications done for consistency, such as adding the shell
prompt for command line examples and trailing slash for directories.
Also corrects the module include header and fixes a few grammar
issues that I introduced.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>