drm/doc: Fix TTM acronym

The TTM acronym is defined for the first time in the documentation as
"Translation Table Maps". Afterwards, "Translation Table Manager" is
used as definition.

Fix the first definition to avoid confusion.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220107180230.132666-1-jose.exposito89@gmail.com