doc: fix command interpolation in Julia dialect doc generation
The tblgen invocation in generate_llvm_dialect_docs was written with
backslash-escaped dollar signs, so the variables were never interpolated
and the command literal failed to parse ("special characters ... must be
quoted in commands"), breaking the docs build and with it `binary-dist`
on every platform:
https://buildkite.com/julialang/julia-pr/builds/1916#01a0593c-ebe2-48d7-b95a-73988340b401
Assisted-by: Claude Code (Fable 5)
