Update degrees_of_freedom.md -> make title consistent with other titles

[DRollin]

I just changed "Freedom" to "freedom" to be consistent with the other titles.
(Was there a reason for the capital F?)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.57%. Comparing base (3b081be) to head (0fbba4a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1106   +/-   ##
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  Coverage   93.57%   93.57%           
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  Files          39       39           
  Lines        6071     6071           
=======================================
  Hits         5681     5681           
  Misses        390      390           

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[KnutAM]

Was there a reason for the capital F?

To follow the DoF abbreviation or header capitalisation, I guess. But that is inconsistent throughout the documentation, so good to make consistent!

At the bottom, the title is Ordering of Dofs which could/should be changed then as well? (we don't use the "first letter of each header word" capitalisation convention on most other pages AFAIK)

[DRollin]

Good point.
Should it be "dofs" or "DOFs" since it is an abbreviation?
I have teaching soon, but after that, I could go through all titles (and occurrances of "dof") and make everything consistent.