typo fix (#859)

Author: fgeorgatos

cwltool.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 from __future__ import absolute_import
-"""Convienance entry point for cwltool.
+"""Convenience entry point for cwltool.
 
 This can be used instead of the recommended method of `./setup.py install`
 or `./setup.py develop` and then using the generated `cwltool` executable.