fix: doc: canbus: fix tx callback function name typo in code snippet

In the `can_send()` example code snippet, it passed the
`can_tx_callback_t` parameter as `tx_irq_callback` even though the
function name was previously defined in the snippet as `tx_callback`.
The parameter passed has been updated to `tx_callback` to maintain
consistency with existing code.

Signed-off-by: Kush Nayak <kushnayak123@gmail.com>