Update isolates.md: Robust Example Fix

[maryamshal]

Fix robust example inconsistency. The constructor signature changes order of parameters mid example which causes issues for a person following along step by step.

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[MaryaBelanger]

Hi @maryamshal, thanks for fixing this! Could you also update the code excerpt source for each of these changes (that's why the build is failing, both the markdown page and the source for example code need to be updated).

You can find the source of a code block right above it usually, indicated by the <?code excerpt ... directive. So on lines 546 and 664 you can see that the source corresponding to the changes you made are in:

• examples/concurrency/lib/robust_ports_example/start.dart line 21, and
• examples/concurrency/lib/robust_ports_example/spawn_2.dart line 33

Make the same changes there and we should be all set! (might help to follow the instructions in the readme too, but it might not be necessary).

If you're having trouble just let us know and we can take over that part for you, Thanks!