When I discovered the right key, I tried to find the right way to contribute to the manual, but on the page http://www.rockbox.org/twiki/bin/view/Main/ManualHowto#How_may_I_help I got lost at the Patch creation. Patch for what?!? Where should I send it?
The manual is written using LaTeX, a typsetting program. It uses plain text files as input that are formatted using a special markup. You can retrieve the source files from svn, and if you're capable of making changes to that sources this is where you should create a patch out of.
If you can't do that directly you can also submit a plain text file (no word processors please) including the new wording (and a notice where it should be located). In both cases the submission goes to our tracker (linked on the left-sided menu) and put in the "manual" category.
Since it's a small change, maybe some of the experts here will do this change for me, or teach me how to add things to the manual 
If you want to contribute to the manual you should get yourself some basic knowledge about Latex (in case you don't know it). Then it shouldn't be too hard to make changes to the manual directly. Feel free to look around on IRC -- usually there are a few people around who can help you in starting to work at the manual and answering questions.