[README] Need to fix Slack badge

[aminoxix]

Description

README.md is the first file one should read when starting a new project. For attractive look, we added some badges also.
But, there's a Slack badge, need to fix that.

Example Screenshot :-

Current Behavior

Desired Behavior

Environment:

• OS: any [e.g. Ubuntu]
• Browser: any [e.g. Chrome, Safari]
• Version: any [e.g. 22]
• Device: any [e.g. laptop, iPhone 8]