[README] Fix downloads count badge

[pricklypointer]

Description

Fixed the download count badge using https://juliapkgstats.com/. The current badge is broken because the old API and PkgVisBoard is not maintained.

Fixes #(issue)

Replaces the broken badge with a new badge that uses the new API

How Has This Been Tested?

N/A

Checklist:

• My code follows the style guidelines of this project. Please run using JuliaFormatter; format(".") in the base directory of the repository (~/.julia/dev/ITensors) to format your code according to our style guidelines.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that verify the behavior of the changes I made.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.

[mtfishman]

Thanks!