docs: use queryByText in guide-disappearance.mdx

[davidjb]

This adjusts the first example on this page to use the queryBy finder instead of getBy to avoid throwing in the query itself.

This gotcha gets explained a few times later in the doc (https://testing-library.com/docs/guide-disappearance#waiting-for-disappearance) but this is the first example you come to, and the use of getByText means the expect(...).toBeInTheDocument() will never be called.

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[davidjb]

Self-closing on account of the explanation at https://github.com/testing-library/eslint-plugin-testing-library/blob/main/docs/rules/prefer-presence-queries.md - the docs would benefit from distinguishing explicit choices on using getBy, assuming that applies here.