updated nix client package addition docs

[shivamashtikar]

Problem

Describe the problem you are trying to solve here

• integrating nix client in haskel repo, but the doc shared contains older url from bitbucket

Solution

Provide a brief summary of your solution so that reviewers can understand your code

• updated nix package addition docs

Environment variable changes

• NA
  What ENVs need to be added or changed
• NA

Pre-deployment activity

Things needed to be done before deploying this change (if any)

• NA

Post-deployment activity

Things needed to be done after deploying this change (if any)

• NA

API changes

Endpoint	Method	Request body	Response Body
API	GET/POST, etc	request	response

Possible Issues in the future

Describe any possible issues that could occur because of this change