samples: drivers: can: Rename directory for consistency

This commit renames the CAN sample directory name from `CAN` to `can`
to be consistent with others.

Noting that the CAN driver test directory is named `tests/drivers/can`,
we have no excuse for naming the CAN driver sample directory
`samples/drivers/CAN`.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>