selftests: mptcp: connect: Use more formal wording in error message

Update error message in mptcp_connect.sh to use a more formal and
consistent wording. The phrase "not able to" was replaced with "Unable to"
for better clarity.

Signed-off-by: Marcelo Moreira <[email protected]>

Author: meritissimo1

tools/testing/selftests/net/mptcp/mptcp_connect.sh

@@ -261,8 +261,8 @@ check_mptcp_disabled()
 
 	# mainly to cover more code
 	if ! ip netns exec ${disabled_ns} sysctl net.mptcp >/dev/null; then
-		mptcp_lib_pr_fail "not able to list net.mptcp sysctl knobs"
-		mptcp_lib_result_fail "not able to list net.mptcp sysctl knobs"
+		mptcp_lib_pr_fail "Unable to list net.mptcp sysctl knobs"
+		mptcp_lib_result_fail "Unable to list net.mptcp sysctl knobs"
 		ret=${KSFT_FAIL}
 		return 1
 	fi