Update expo installation command to npx expo in docs

[amandeepmittal]

Description

One Line Summary

REQUIRED - Very short description that summaries the changes in this PR.

This PR updates the expo command to npx expo since the global Expo CLI is in legacy and is replaced by the new local CLI.

Details

Motivation

REQUIRED - Why is this code change being made? Or what is the goal of this PR? Examples: Fixes a specific bug, provides additional logging to debug future issues, feature to allow X.

As per the latest Expo SDK (46 and above), we released a new local CLI and recommend to use npx expo to perform tasks such as installing a config plugin, a library, starting the development server, and so on. For more info, please see Expo CLI docs here.

Updating the docs so that anyone using this config plugin doesn't have to switch between legacy and the new CLI.

Scope

RECOMMEND - OPTIONAL - What is intended to be effected. What is known not to change. Example: Notifications are grouped when parameter X is set, not enabled by default.

Only the documentation.

OPTIONAL - Other

OPTIONAL - Feel free to add any other sections or sub-sections that can explain your PR better.

Testing

Manual testing

REQUIRED - Explain what scenarios were tested and the environment.

Running the command with npx expo.

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have personally tested this on my device, or explained why that is not possible
• I have tested this on the latest version of the plugin
• I have tested this on both Android and iOS, or explained why that is not possible

Final pass

• Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

[brismithers]

Thank you for your contribution!