Fixes grammatical error

these is -> these are

Author: anauleau

views/md/introduction.md

@@ -49,7 +49,7 @@ Then, this JSON is **Base64Url** encoded to form the first part of the JWT.
 The second part of the token is the payload, which contains the claims. Claims are statements about an entity (typically, the user) and additional metadata.
 There are three types of claims: *reserved*, *public*, and *private* claims.
 
-- **Reserved claims**: These is a set of predefined claims which are not mandatory but recommended, to provide a set of useful, interoperable claims. Some of them are: **iss** (issuer), **exp** (expiration time), **sub** (subject), **aud** (audience), and others.
+- **Reserved claims**: These are a set of predefined claims which are not mandatory but recommended, to provide a set of useful, interoperable claims. Some of them are: **iss** (issuer), **exp** (expiration time), **sub** (subject), **aud** (audience), and others.
 
 	> Notice that the claim names are only three characters long as JWT is meant to be compact.