update rationalize docs (#23541)

Removed description of default for T. Was redundant with signature

Author: Christian Kurz

base/rational.jl

@@ -114,7 +114,6 @@ widen(::Type{Rational{T}}) where {T} = Rational{widen(T)}
 
 Approximate floating point number `x` as a [`Rational`](@ref) number with components
 of the given integer type. The result will differ from `x` by no more than `tol`.
-If `T` is not provided, it defaults to `Int`.
 
 ```jldoctest
 julia> rationalize(5.6)