The invention proposes an adapter (26) which is delimited by two external lateral faces and which is shaped so as to be mounted in a cap (28) of reverse U-shaped section, comprising a pair of front locking lugs (44a, 44b) which cooperate with the associated notches (59a, 59b) of the adapter (26) in order to fix the cap (28) to the adapter (26) in a dismantlable manner, each of the lateral faces (29a, 29b) of the body (25) of the adapter (26) comprising a front locking notch (59a, 59b), each thereof being capable of receiving an associated front locking lug (44a, 44b) of the cap (28), and the body (25) of the adapter (26) comprising rear locking notches which are arranged at one end of a resilient branch (62a, 62b) of the adapter.