fix(installdashboard): add quotes around dashboard installation

[garrettbates]

Context

Copy pasting the dashboard installation command resulted in this error:

pip install salesforce-merliondashboard 
ERROR: Could not find a version that satisfies the requirement salesforce-merliondashboard (from versions: none)
ERROR: No matching distribution found for salesforce-merliondashboard

Change

Updated readme to put the dashboard installation command in quotes to avoid the error:

pip install "salesforce-merlion[dashboard]"

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @garrettbates to sign the Salesforce Inc. Contributor License Agreement.