Rewrite the text in Muscle modeling lesson 6 (#10)

* Rewrite the text in Muscle modeling lesson 6

* Update formatting and line breaks

* Update Muscle_modeling/lesson6.md

Co-authored-by: M. Damsgaard <67595823+mdabt@users.noreply.github.com>
Signed-off-by: Morten Enemark Lund <melund@gmail.com>

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Signed-off-by: Morten Enemark Lund <melund@gmail.com>
Co-authored-by: M. Damsgaard <67595823+mdabt@users.noreply.github.com>

Author: melund