dma-buf-map: Fix dot vs comma in example

Fix typo: separate arguments with comma rather than dot.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111003305.1214667-1-lucas.demarchi@intel.com