docs: remove X from kind

[tenstad]

It's unclear how one should name XRD kinds. Given the following: https://github.com/crossplane/docs/blob/3b8bad4228132f97aaf6b51a316fa19c1e76f76c/content/v2.0/get-started/get-started-with-composition.md#L128-L134 should the kinds within composite-resource-definitions.md also be rewritten to remove X?

If not, feel free to just close this PR.

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