Show correct format documentation

Shows the `Dates.format` table which was the behaviour in the Julia 0.6
documentation.

Author: omus

stdlib/Dates/docs/src/index.md

@@ -654,7 +654,7 @@ Dates.DateTime(::Dates.Period)
 Dates.DateTime(::Function, ::Any...)
 Dates.DateTime(::Dates.TimeType)
 Dates.DateTime(::AbstractString, ::AbstractString)
-Dates.format
+Dates.format(::Dates.TimeType, ::AbstractString)
 Dates.DateFormat
 Dates.@dateformat_str
 Dates.DateTime(::AbstractString, ::Dates.DateFormat)

stdlib/Dates/src/io.jl

@@ -34,7 +34,7 @@ All subtypes of `AbstractDateToken` must define this method in order
 to be able to print a Date / DateTime object according to a `DateFormat`
 containing that token.
 """
-function format end
+format(io::IO, tok::AbstractDateToken, dt::TimeType, locale)
 
 # fallback to tryparsenext/format methods that don't care about locale
 @inline function tryparsenext(d::AbstractDateToken, str, i, len, locale)