Update forecast.py comment typo

[aperetti]

Fix minor typo in comment from DHI to GHI

pvlib python pull request guidelines

Thank you for your contribution to pvlib python! You may delete all of these instructions except for the list below.

You may submit a pull request with your code at any stage of completion.

The following items must be addressed before the code can be merged. Please don't hesitate to ask for help if you're unsure of how to accomplish any of the items below:

• Closes issue #xxxx
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes LGTM and SticklerCI checks.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

Brief description of the problem and proposed solution (if not already fully described in the issue linked to above):

[wholmgren]

thanks @aperetti!