tidy file path in help text for invocation of docnn.json example

pytext/docs/source/train_your_first_model.rst

@@ -18,7 +18,7 @@ You can use PyText as a library either in your own scripts or in a Jupyter noteb
 
     Example:
 
-      pytext train < demos/docnn.json
+      pytext train < demo/configs/docnn.json
 
   Options:
     --config-file TEXT

pytext/main.py

@@ -191,7 +191,7 @@ def main(context, config_file, config_json, config_module, include):
 
     Example:
 
-      pytext train < demos/docnn.json
+      pytext train < demo/configs/docnn.json
     """
     for path in include or []:
         # remove possible trailing / from autocomplete in --include