As a Scrum Master, you should encourage everyone to broaden their knowledge regarding:
- Engineering Practices
Testing, Refactoring, Emergent Design, Automation and many others are all essential to build the product right - all of these are easy to learn, but extremely difficult to master.
- Product Domain
Developers who are only technical experts will build the wrong product, unless they actively strive to understand the domain of the product they develop. Low hanging fruit may be out of your reach when you can't make the right connections.
The ideal of infinite Return on Investment requires responding to demand without any delay. Every step you take to be more agile will pay off - but what steps are available to you?
The team must take steps to actively shape the organization in a positive fashion. To do this, they must understand well how their organization and the world around them "ticks".
Since "knowledge" itself is so broad, there is no definite way to approach broadening knowledge. You may read books, share experiences with your team or others in the organization - build up Communities of Practice, join Special Interest Groups or whatever you see fit. You need to make sure is that people are thirsty for more knowledge. You can't "push" knowledge. People must want it.