Fix residual reference to version 6 in README
Fix residual reference to version 6 in README

The suggested value for OPENEDX_EXTRA_PIP_REQUIREMENTS in the README
still contained a reference to version 6. Fix it to refer to version 7
instead.

hastexo/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock