fix hierarchy level of "API reference" in `Dates` documentation (#55483)
Currently, "API reference" is at the same level as "Dates" although it
is a subsection of it. This looks particularly weird in the PDF version
of the manual: Section 67 is "Dates" and Section 68 is "API reference".
Note that I didn't change the nesting level of the subsection
"Constants" at the end of the file. As a result, it is now at the same
level as "Dates and Time Types" and "Dates Functions". Before it was a
subsection of the latter, which appears wrong to me.