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.

(cherry picked from commit 881be64)

stdlib/Dates/docs/src/index.md

@@ -684,9 +684,9 @@ value in the days field is uncertain.
 See the [API reference](@ref stdlib-dates-api) for additional information
 on methods exported from the `Dates` module.
 
-# [API reference](@id stdlib-dates-api)
+## [API reference](@id stdlib-dates-api)
 
-## Dates and Time Types
+### Dates and Time Types
 
 ```@docs
 Dates.Period
@@ -701,7 +701,7 @@ Dates.TimeZone
 Dates.UTC
 ```
 
-## Dates Functions
+### Dates Functions
 
 ```@docs
 Dates.DateTime(::Int64, ::Int64, ::Int64, ::Int64, ::Int64, ::Int64, ::Int64)
@@ -730,7 +730,7 @@ Dates.now(::Type{Dates.UTC})
 Base.eps(::Union{Type{DateTime}, Type{Date}, Type{Time}, TimeType})
 ```
 
-### Accessor Functions
+#### Accessor Functions
 
 ```@docs
 Dates.year
@@ -758,7 +758,7 @@ Dates.monthday
 Dates.yearmonthday
 ```
 
-### Query Functions
+#### Query Functions
 
 ```@docs
 Dates.dayname
@@ -777,7 +777,7 @@ Dates.quarterofyear
 Dates.dayofquarter
 ```
 
-### Adjuster Functions
+#### Adjuster Functions
 
 ```@docs
 Base.trunc(::Dates.TimeType, ::Type{Dates.Period})
@@ -797,7 +797,7 @@ Dates.tonext(::Function, ::Dates.TimeType)
 Dates.toprev(::Function, ::Dates.TimeType)
 ```
 
-### Periods
+#### Periods
 
 ```@docs
 Dates.Period(::Any)
@@ -808,7 +808,7 @@ Dates.default
 Dates.periods
 ```
 
-### Rounding Functions
+#### Rounding Functions
 
 `Date` and `DateTime` values can be rounded to a specified resolution (e.g., 1 month or 15 minutes)
 with `floor`, `ceil`, or `round`.
@@ -837,7 +837,7 @@ Dates.date2epochdays
 Dates.datetime2epochms
 ```
 
-### Conversion Functions
+#### Conversion Functions
 
 ```@docs
 Dates.today