Update graphs and link in the secondary cache blog post (#8348)

Summary:
Update graphs to remove FB specific terms such as WSF, and update link to the Github issue in the secondary cache blog post.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8348

Reviewed By: ramvadiv

Differential Revision: D28773858

Pulled By: anand1976

fbshipit-source-id: 86281d5c6928550d68d5aa66aae39a41a41f928f

docs/_posts/2021-05-27-rocksdb-secondary-cache.markdown

@@ -191,5 +191,5 @@ Longer term, we plan to deploy this in production at Facebook.
 
 ## Call to Action
 
-We are hoping for a community contribution of a secondary cache implementation, which would make this feature usable by the broader RocksDB userbase. If you are interested in contributing, please reach out to us in this issue.
+We are hoping for a community contribution of a secondary cache implementation, which would make this feature usable by the broader RocksDB userbase. If you are interested in contributing, please reach out to us in [this issue](https://github.com/facebook/rocksdb/issues/8347).