documentation: coccinelle: Improve command example for make C={1,2}

Modify coccinelle documentation to further clarify
the usage of the makefile C variable by coccicheck.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Acked-by: Julia Lawall <julia.lawall@inria.fr>
Link: https://lore.kernel.org/r/20200811002350.5553-1-sylphrenadin@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>