sigs.md in rbs-1.2.1

- old
+ new
@@ -129,11 +129,13 @@
 `RBS_TEST_OPT` is to pass the options for rbs handling.
 You may need to specify `-r` or `-I` to load signatures.
 The default is `-I sig`.
 
 ```
-RBS_TEST_OPT='-r set -r pathname -I sig'
+RBS_TEST_OPT='-r pathname -I sig'
 ```
+
+Replacing `pathname` with the `stdlib` you want to include. For example, if you need to load `Set` and `BigDecimal` in `stdlib`, you would need to have `RBS_TEST_OPT='-r set -r bigdecimal -I sig'`
 
 `RBS_TEST_LOGLEVEL` can be used to configure log level. Defaults to `info`.
 
 `RBS_TEST_RAISE` may help to debug the type signatures.
 If the environment variable is set, it raises an exception when a type error is detected.