Fixed type where docs said migrator.py but the file is run_migrator.py (Submitty#176)

Author: elihschiff

_docs/developer/migrations.md

@@ -62,7 +62,7 @@ database (for course migrations).
 To see additional options, run:
 
 ```
-sudo python3 /usr/local/submitty/GIT_CHECKOUT/Submitty/migration/migrator.py -h
+sudo python3 /usr/local/submitty/GIT_CHECKOUT/Submitty/migration/run_migrator.py -h
 ```