[Docs] Make it easier to install the Reader SDK

[gjreasoner]

Summary

It's a minor change that hopefully doesn't set too many people back, but just adding the dollar sign to the beginning of the variables here would enable people to copy their credentials in the terminal first and then just copy and paste the install command.

To go one step further, it'd be really nice to have the Reader SDK page include these changes.

Here is what installation history would then look like:

➜  ios git:(master) ✗ YOUR_SQUARE_READER_APP_ID=sq0idp...
➜  ios git:(master) ✗ YOUR_SQUARE_READER_REPOSITORY_PASSWORD=x6yflacmlhjja....
➜  ios git:(master) ✗ ruby <(curl https://connect.squareup.com/readersdk-installer) install \
    --app-id $YOUR_SQUARE_READER_APP_ID                                    \
    --repo-password $YOUR_SQUARE_READER_REPOSITORY_PASSWORD

For those in the know about the bash trick, the time savings is nice, and for those not in the know, it should not have any effect.

Thanks for the great SDK, can't wait to get started building some cool stuff.

Changelog

• Updated installation instructions for the Reader SDK

Test Plan

[CLAassistant]

All committers have signed the CLA.

[mansi-square]

Created another PR for this.