Update runningthetool.md

Give example for -D with toys

docs/part3/runningthetool.md

@@ -204,7 +204,7 @@ If you are using `toysFrequentist`, be aware that the values set by `--setParame
 
 ### Generate only
 
-It is also possible to generate the toys first, and then feed them to the methods in <span style="font-variant:small-caps;">Combine</span>. This can be done using `-M GenerateOnly --saveToys`. The toys can then be read and used with the other methods by specifying `--toysFile=higgsCombineTest.GenerateOnly...` and using the same options for the toy generation.
+It is also possible to generate the toys first, and then feed them to the methods in <span style="font-variant:small-caps;">Combine</span>. This can be done using `-M GenerateOnly --saveToys`. The toys can then be read and used with the other methods by specifying `--toysFile=higgsCombineTest.GenerateOnly...` and using the same options for the toy generation. You can specify to run on a single toy, in place of the observed data, by including the option `-D file.root:toys/toy_i`. For example adding `-D higgsCombineTest.GenerateOnly.mH120.123456.root:toys/toy_10` will run on the 10th toy that was generated and saved in the file `higgsCombineTest.GenerateOnly.mH120.123456.root`. 
 
 !!! warning
     Some methods also use toys within the method itself (eg `AsymptoticLimits` and `HybridNew`). For these, you should **not** specify the toy generation with `-t` or the options above. Instead, you should follow the method-specific instructions.