Docs 2.5: Fix floating groups heading level (2nd try)

The first attempt successfully pushed the Floating Groups chapter to
its proper place in the table of contents, but it also mistakenly pushed
many following chapters down one heading level. This commit follows the
approach already common in the root.txt file, and also undoes the
previous approach of shifting the heading level within the chapter file
itself. I suspect the previous approach only failed because it lacked a
blank line before the final 'leveloffset' command, but this new
approach is better because it is already in common use.

Signed-off-by: Remington Steed <rjs7@calvin.edu>