Docs: remove --curriculum flag (#3921)

docs/Training-ML-Agents.md

 #### Training with a Curriculum
 
 Once we have specified our metacurriculum and curricula, we can launch
-`mlagents-learn` using the `–curriculum` flag to point to the config file for
+`mlagents-learn` using the config file for
 our curricula and PPO will train using Curriculum Learning. For example, to
 train agents in the Wall Jump environment with curriculum learning, we can run: