Fix Heading Levels in Tutorials

[ntjohnson1]

We should only have 1 top level heading for every tutorial # everything else should be a sub-heading ##, ###, .. etc. See the tutorials landing page where it gives a preview of the nested heading structure https://pyttb.readthedocs.io/en/latest/tutorials.html on a quick glance it looks like cp_als, and hosvd have this issue.

Should be a fairly small change so might be nice to wrap up while you are working on the matplotlib fixes.

[jeremy-myers]

This will be resolved by PR https://github.com/sandialabs/pyttb/pull/276

[ntjohnson1]

Resolved in #276