[jfx19] RFR: 8286678: Fix mistakes in FX API docs [v3]

[Kevin Rushforth]

On Thu, 18 Aug 2022 23:50:03 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

> ```
> fatal: ambiguous argument 'upstream/jfx19': unknown revision or path not in the working tree.
> ```

I should have said "presuming you have a remote configured with the name `upstream`". Anyway, what you did was good.

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PR: https://git.openjdk.org/jfx/pull/877