Fix minor typo in the sample configuration file
# WARNING:
# Do not use this file as is! It only lists and documents available options
# without caring about their consistency. Prefer using "opennmt-defaults.yml" as a
-# base configuration and specialize it with addtional configuration files.
+# base configuration and specialize it with additional configuration files.
# The directory where models and summaries will be saved. It is created if it does not exist.
model_dir: toy-ende
