Section depth for printer-friendly output off-by-one error

Proper subsections of a top-level section in printer-friendly output have level equal to the parent.

That is, we have

Structure:     Expected depth  Actual depth
section 1           1                      1
section 1.1         2                      1
  section 1.1.1     3                      2
section 1.2         2                      1

The attached patch is a one-line change. It increments $depth before calling book_print_recurse() from book_print().

I'm marking this as critical since it is interfering with generation of PDF from the Handbook.

Djun