Update demo doc for waitForElementVisible - fix access via page objec…

[michaelleb]

…t - .api. is not needed

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[garg3133]

@michaelleb Thanks for this PR!

Would you be able to make the same changes for other waitFor* commands as well: waitForElementPresent, waitForElementNotPresent and waitForElementNotVisible?

Also, please sign the CLA so that we can merge this PR.

Thanks again!

[garg3133]

This will be fixed in #4210. Thanks for your PR!