[doc][doxygen][img] cleanup tpfa and mpfa figures

What this MR does / why does DuMux need it:

As stated in the dumux-course issue #77, there were two points that need to be fixed for the tpfa and the mpfa figure:

• for tpfa: adjusting distance vector to point in middle of face
• for mpfa: use same notation as for tpfa (x instead of u)

Notes for the reviewer

TODO: insert text here

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• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.