Fix stale check_model doctest broken by the #1438 fence fix

[shravanngoswamii]

main's Doctests workflow is currently red: https://github.com/TuringLang/DynamicPPL.jl/actions/runs/29509495151/job/87658917625

#1438 fixed a real markdown bug (an unclosed ``` fence in check_model's docstring), which is good, but closing that fence changed where Documenter thinks the jldoctest block ends. That exposed a doctest that was already stale: the "Incorrect model" example expected

julia> check_model(model; error_on_failure=true)
ERROR: varname x used multiple times in model

but check_model's actual behavior for repeated VarNames (since at least the DebugAccumulator rework) is to @warn per repeated variable and then raise the generic error_on_failure message at src/debug_utils.jl:265-266:

julia> check_model(model; error_on_failure=true)
┌ Warning: Assigning to the variable x led to a previous value being overwritten. This indicates that a value is being set twice (e.g. if the same variable occurs in a model twice).
└ @ DynamicPPL.DebugUtils DynamicPPL.jl/src/debug_utils.jl:237
ERROR: Model check failed; please see the warnings above for details.

Updated the docstring to match, mirroring the same warning-block style already used in the "Correct model" example just above it. Verified locally: GROUP=Doctests julia --project=. -e 'using Pkg; Pkg.test()' passes (1/1) on Julia 1.12, matching the CI job that was failing.

[github-actions]

DynamicPPL.jl documentation for PR #1439 is available at:
https://TuringLang.github.io/DynamicPPL.jl/previews/PR1439/