Update README.md

Use https

Author: akashhansda

README.md

@@ -3,7 +3,7 @@
 **main branch represents 6.x line**. See the [upgrading to 4.x doc](docs/upgrading-to-4-0.md), [upgrading to 5.x doc](docs/upgrading-to-5-0.md), or [upgrading to 6.x doc](docs/upgrading-to-6-0.md) for instructions on how to upgrade. Bug fixes should go in the 5.x branch for now.
 
 The gem will automatically apply several headers that are related to security.  This includes:
-- Content Security Policy (CSP) - Helps detect/prevent XSS, mixed-content, and other classes of attack.  [CSP 2 Specification](http://www.w3.org/TR/CSP2/)
+- Content Security Policy (CSP) - Helps detect/prevent XSS, mixed-content, and other classes of attack.  [CSP 2 Specification](https://www.w3.org/TR/CSP2/)
   - https://csp.withgoogle.com
   - https://csp.withgoogle.com/docs/strict-csp.html
   - https://csp-evaluator.withgoogle.com