doc: name the pflag types the wrappers actually hold

[ump45nose]

Overview:
Two comments in flags.go named pflag types that do not exist. pflagValueSet was described as "a wrapper around *pflag.ValueSet" while the struct holds *pflag.FlagSet (flags.go:23), and pflagValue as a wrapper around *pflag.flag while the field is flag *pflag.Flag (flags.go:36). Both comments now name the real types. Comment text only; no code changed.

What problem does it solve?:
There is no pflag.ValueSet in the pinned dependency — go.mod:12 requires github.com/spf13/pflag v1.0.10, which exports FlagSet, Flag, Value and SliceValue, and searching it for type ValueSet turns up nothing. The only way a caller obtains a pflagValueSet is through BindPFlags(flags *pflag.FlagSet) (viper.go:1087, viper.go:1091), which wraps the FlagSet, and VisitAll iterates it as p.flags.VisitAll(func(flag *pflag.Flag) …) (flags.go:27-31). So the comment points readers at a type they cannot find, one line above the type that is actually used.

Special notes:

• gofmt is unaffected: line comments only, no struct tags or alignment touched.
• I could not run go build ./..., go vet ./... or go test ./... — this environment's shell sandbox refused all toolchain commands, so CI is the first real run. The claim is settled statically by reading flags.go:20-37 against viper.go:1087-1092 and the vendored pflag v1.0.10.
• AI assistance disclosure: an autonomous agent identified the stale type names, made the edit, and wrote this description on @ump45nose's behalf.