Remove dated "beta" text in README (#1499)

Summary:
Pull Request resolved: #1499

Remove the dated "beta" text in our README.

Reviewed By: cyrjano

Differential Revision: D69695282

fbshipit-source-id: b4278597ff27c991be04cbf5c0de965fbcc43c63

Author: craymichael

README.md

@@ -25,8 +25,6 @@ of integrated gradients, saliency maps, smoothgrad, vargrad and others for
 PyTorch models. It has quick integration for models built with domain-specific
 libraries such as torchvision, torchtext, and others.
 
-*Captum is currently in beta and under active development!*
-
 
 #### About Captum