Update index-de.html

Some tweaks to the German translation:

* "don't be a jerk" was translated as "be an adult", now: "behave like an adult"
* "body size" was translated as "body height", but I believe what is meant is how skinny or not a person is? If so, now I translated it as "body shape"
* fixed a few grammar glitches and typos.
Merge pull request #20 from fwenzel/patch-1

Update index-de.html