doc: rename `sparam_syms` to `slot_syms`

[t-bltg]

sparam_syms was removed in https://github.com/JuliaLang/julia/pull/31015.

[vtjnash]

Oh, I see that diff looks confusing. No, sparam_syms is gone, without direct replacement, as the information is not useful post-lowering. It can probably be reconstructed from sig however.

[t-bltg]

Thanks, should I close this and I leave you fix the ambiguity instead ?

[ViralBShah]

What to do here next?

[t-bltg]

No, sparam_syms is gone, without direct replacement

Let's move on this one, fixed by removing both traces of sparam_syms and slot_syms.