Fix misleading error message for lowerkey

The error message now correctly suggests defining
`StructUtils.lowerkey(::JSON.JSONStyle, ::T)` instead of
`StructUtils.lowerkey(::T)` which doesn't work.

Fixes #414

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: quinnj

src/write.jl

@@ -251,7 +251,7 @@ end
 StructUtils.lowerkey(::JSONStyle, s::AbstractString) = s
 StructUtils.lowerkey(::JSONStyle, sym::Symbol) = String(sym)
 StructUtils.lowerkey(::JSONStyle, n::Union{Integer, Union{Float16, Float32, Float64}}) = string(n)
-StructUtils.lowerkey(::JSONStyle, x) = throw(ArgumentError("No key representation for $(typeof(x)). Define StructUtils.lowerkey(::$(typeof(x)))"))
+StructUtils.lowerkey(::JSONStyle, x) = throw(ArgumentError("No key representation for $(typeof(x)). Define StructUtils.lowerkey(::JSON.JSONStyle, ::$(typeof(x)))"))
 
 """
     JSON.json(x) -> String