selftests: net: Fix minor typos in MPTCP and psock tests

Fixes minor spelling errors:
- `simult_flows.sh`: "al testcases" -> "all testcases"
- `psock_tpacket.c`: "accross" -> "across"

Signed-off-by: Suchit Karunakaran <suchitkarunakaran@gmail.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250218165923.20740-1-suchitkarunakaran@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>