Fix copy/paste error in help for --pidns

--pidns acts on a pid namespace, not a user namespace.

Resolves: #531
Thanks: hadess
Signed-off-by: Simon McVittie <smcv@collabora.com>

bubblewrap.c

@@ -311,7 +311,7 @@ usage (int ecode, FILE *out)
            "    --unshare-cgroup-try         Create new cgroup namespace if possible else continue by skipping it\n"
            "    --userns FD                  Use this user namespace (cannot combine with --unshare-user)\n"
            "    --userns2 FD                 After setup switch to this user namespace, only useful with --userns\n"
-           "    --pidns FD                   Use this user namespace (as parent namespace if using --unshare-pid)\n"
+           "    --pidns FD                   Use this pid namespace (as parent namespace if using --unshare-pid)\n"
            "    --uid UID                    Custom uid in the sandbox (requires --unshare-user or --userns)\n"
            "    --gid GID                    Custom gid in the sandbox (requires --unshare-user or --userns)\n"
            "    --hostname NAME              Custom hostname in the sandbox (requires --unshare-uts)\n"