Fix texinfo syntax errors

Fix the following texinfo syntax errors:

ledger/doc/ledger3.texi:7878: @itemx must follow @item
ledger/doc/ledger3.texi:7939: @itemx must follow @item
ledger/doc/ledger3.texi:7944: @itemx must follow @item
BUILD_WEB_DOCS implies BUILD_DOCS

This fixes the build when -DBUILD_WEB_DOCS=1 is specified wihout
specifying -DBUILD_DOCS.  This fails because man2html/groff are
not found because the find_program command for these programs
is only run if BUILD_DOCS is enabled.