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

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.

Author: AdamAtri

docs/tooling/testing/end-to-end-testing/overview.md

@@ -37,7 +37,7 @@ For the plugin to work correctly you need to have:
 
 * Install Appium globally:
 ```shell
-$ npm install -g appium
+$ npm install -g appium@1.18.1
 ```
 
 * iOS Dependencies