give file_pretrained_model an example (#56)

Without one, `run_and_check_output` cannot resolve a value for `--output` and
every `*_train` component fails its test with "--output is a required argument".

Author: rcannood

src/api/file_pretrained_model.yaml

@@ -1,3 +1,4 @@
 type: file
+example: "model"
 label: "Pretrained model"
 summary: "A pretrained model for predicting the expression of one modality from another."