docs: TROUBLESHOOTING points at a viper_yaml3 build tag that is gone

[ump45nose]

Overview:

TROUBLESHOOTING.md tells users to pass a viper_yaml3 build tag that does not exist in this module. Reworded the advice so it points at something a user can actually do.

What problem does it solve?:

The last line of the "Unquoted 'y' and 'n' characters" section reads:

Upgrading to YAML v3 (for the time being this is possible by passing the viper_yaml3 tag to your build)

There is no such tag any more. The only build tags in the module are viper_bind_struct and viper_finder:

internal/features/bind_struct_default.go:1://go:build !viper_bind_struct
internal/features/bind_struct.go:1://go:build viper_bind_struct
internal/features/finder.go:1://go:build viper_finder
internal/features/finder_default.go:1://go:build !viper_finder

and YAML v3 is unconditionally the codec Viper uses:

• internal/encoding/yaml/codec.go:3 — import "go.yaml.in/yaml/v3" (no variant file behind a tag)
• go.mod:15 — go.yaml.in/yaml/v3 v3.0.4

So a user following this instruction today adds a tag with no effect and still sees YAML 1.1 boolean resolution. The replacement says the upgrade already happened inside Viper, and points at the two real options: upgrade Viper, or supply a YAML v3 codec through the codec registry (viper.WithCodecRegistry, viper.Codec).

Docs only, one line.

Special notes for a reviewer:

• I did not claim which Viper release made v3 unconditional (the local clone is shallow, so I could not check history) — the wording sticks to what the current tree shows.
• Overlap with my own open docs: point TROUBLESHOOTING at the maintained mapstructure fork #2162, which edits this same file: that PR changes the mapstructure import path in line 5 (## Unmarshaling doesn't work), this one changes line 32 in a different section. gh pr diff 2162 confirms the hunks are ~27 lines apart, so the two are independent and either can land first.
• Testing boundary: no build or test run was possible in this environment (the sandbox refused go build/go test), but the change is markdown text; the cited tag list was produced by searching every //go:build line in the repo.
• AI assistance disclosure: prepared with an AI coding agent; the tag and import facts were re-verified by reading the files, and I am responsible for the result.