fix: correct upsertStructuredProperties and removeStructuredProperties examples

[Abhinav0905]

What

Corrects the two Structured Properties examples in
skills/datahub-enrich/references/mutation-reference.md. As written, neither can
succeed against any GMS — both fail GraphQL validation before reaching execution.

Why

I hit this while building a tiering agent on DataHub for the Agent Hackathon: I
copied the documented upsertStructuredProperties example and spent a while
assuming my own setup was at fault.

Run verbatim against DataHub OSS v1.7.0, the documented upsertStructuredProperties
returns two validation errors:

Validation error (SubselectionRequired@[upsertStructuredProperties]) :
  Subselection required for type 'StructuredProperties!' of field 'upsertStructuredProperties'
Validation error (WrongType@[upsertStructuredProperties]) :
  argument 'input' with value 'ObjectValue{...}' ...

and removeStructuredProperties returns one:

Validation error (SubselectionRequired@[removeStructuredProperties]) :
  Subselection required for type 'StructuredProperties!' of field 'removeStructuredProperties'

Three distinct problems:

1. The input field is structuredPropertyInputParams, not structuredPropertyInputs.
2. values is [PropertyValueInput!]! — each value is an object ({ stringValue: "..." }
   or { numberValue: 4.0 }), not a bare string.
3. Both mutations return StructuredProperties! and therefore require a selection set.

Verification

Both corrected forms return zero validation errors against a live GMS v1.7.0
(they proceed to execution and fail only on the deliberately non-existent probe URN
I used, which is the expected behaviour for an unknown asset).

One extra note added

A sentence documenting a silent failure that cost me the most time: sending
stringValue to a NUMBER-typed property is accepted by GraphQL and then dropped
server-side — no error returned, no value stored, and no new aspect version. It is
indistinguishable from search-index lag until you notice the value never changes.
Worth one line in the reference so the next person does not have to work it out.

Docs-only change; no behaviour affected.