typo in docu of fourth order unit tensor (SymmetricTensor<4, dim, Number> identity_tensor())

[jfriedlein]

Hello,

I found a minor typo in the documentation of SymmetricTensor<4, dim, Number> identity_tensor(); (docu of identity_tensor). In the index notation (line 101-102) the last index pair for \mathcal{I}_{ijkl} should be "jk" and not "jl". (compare for instance Wriggers "Nonlinear Finite Element Methods" p.79 eq. (3.270) or Notes on Continuum Mechanics - Tensors eq. (1.175)) This appears to be a copy-paste error from the previous line 101 that sneaked in with #9830.

Obviously the implementation is fine, it is just this one character in the documentation that I stumble over from time to time.
Normally, such a small thing would not bother me, but this documentation is so well written and very helpful that I often look things up there instead of opening a book. Overall, a fantastic work that is very much appreciated!
I took this opportunity to try a pull request, so please let me know if anything is out of order.

[jppelteret]

Well spotted! Thanks for fixing this @jfriedlein.

[masterleinad]

We require all commits to be associated with a proper first and last name:

shell: /bin/bash -e {0}
invalid author 'jfriedlein' without firstname and lastname

hint: for possible solutions, consult the webpage:
      dealii/dealii/wiki/Indentation#commit-authorship

Would you mind fixing this by following the instructions in the link?