tty-ask-pwd-agent: fix message forwarded to wall(1)

Commit a1c111c wrongly replaced '!' with ':' when the message is
forwarded to wall(1).

Indeed in this case we are not requesting the user for providing a password but
instead we are simply displaying a message to suggest starting
tty-ask-password-agent for providing the password.

src/tty-ask-password-agent/tty-ask-password-agent.c

@@ -322,7 +322,7 @@ static int parse_password(const char *filename, char **wall) {
 
                 if (asprintf(&_wall,
                              "%s%sPassword entry required for \'%s\' (PID %u).\r\n"
-                             "Please enter password with the systemd-tty-ask-password-agent tool:",
+                             "Please enter password with the systemd-tty-ask-password-agent tool.",
                              strempty(*wall),
                              *wall ? "\r\n\r\n" : "",
                              message,