Remove a spurious s (#384)

Author: yoavweiss

README.md

@@ -1,6 +1,6 @@
 # Permissions Policy and Document Policy
 
-## Permisssions Policy
+## Permissions Policy
 
 A web platform API which gives a website the ability to allow and deny the use of browser features in its own frame, and in iframes that it embeds. Examples of [features](https://github.com/w3c/webappsec-feature-policy/blob/master/features.md) that could be controlled by permissions policy include: