part: correct documentation of part_get_bootable()

We have to use 'Return:' to render the description of the return value in
the HTML documentation.

Fixes: f55aa445 ("part: Add a fallback for part_get_bootable()")
Fixes: dcffa442 ("part: Add a function to find the first bootable partition")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>