Update documentation to match example

[cfra]

In commit 5d96a5b the example was
updated to use a SHA256 fingerprint.

The documentation should reflect that.

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?

The following probably doesn't apply for docs:

• Have you written new tests for your changes?
• Does your submission pass tests?

The failed Travis check seems to be unrelated.

• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[coveralls]

Coverage remained the same at 68.381% when pulling 8fee8c9 on cfra:feature/documentation-nit into 48fb95c on IdentityPython:master.

[leifj]

Turns out the documentation isn't all that misleading but the example is - currently only sha1 fingerprints are supported. This is clearly something that needs to get fixed.