core: Document the right type for 'str*' in the RPC interface

doc/rpc/ser_rpc.xml

@@ -377,7 +377,7 @@ add("sd", string_param, int_param);
 			<row>
 			    <entry>String</entry>
 			    <entry>S</entry>
-			    <entry>str</entry>
+			    <entry>str*</entry>
 			</row>
 			<row>
 				<entry>Optional modifier</entry>