siputils: docs - added missing parenthesis in example

src/modules/siputils/doc/siputils_admin.xml

@@ -654,7 +654,7 @@ if (is_uri("$avp(i:705)") {
 if (is_tel_number("$rU")) {  # Test if R-URI user is telephone number
    ...
 }
-if (is_tel_number("+24242424") {
+if (is_tel_number("+24242424")) {
    ...
 }
 ...