[video_player] Fix small error in example

[ndahlquist]

VideoPlayerValue does not have an initialized property. The correct property is isInitialized

Test Plan

• Copy the example code into a Flutter project, verify it compiles and runs correctly.

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[ndahlquist]

@googlebot I signed it!

[stuartmorgan]

Thanks for the contribution, but this is already being fixed in #3713, so I'm going to close this in favor of continuing with that PR.