docs: Update e2e testing overview.md

[AdamAtri]

doc(e2e-testing-overview): add global appium ver

The current version of appium on npm is 1.19.0, which is incompatible with nativescript-dev-appium@6.1.3 and lower (more specifically if fails to verify the signature of appium-uiautomator2-server-v4.15.0.apk). Appending the version to the global install prevents this incompatibility error.

PR Checklist

• [x ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
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What is the current state of the documentation article?

Current appium version on npm is incompatible with appium-uiautomator2-server-v4.15.0.apk

What is the new state of the documentation article?

Instructs the user to install appium@1.18.1 rather than appium latest

Fixes/Implements/Closes #[Issue Number].

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[AdamAtri]

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[AdamAtri]

@cla-bot check

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