Fix documentation (#142)

* Fix documentation

* Fix types in documentation

README.md

@@ -54,12 +54,12 @@ The claims fields which can be validated. (see [validation](#validation))
 ```rust
 #[derive(Debug, Serialize, Deserialize)]
 struct Claims {
-    aud: String         // Optional. Audience
-    exp: DateTime<Utc>, // Required (validate_exp defaults to true in validation). Expiration time
-    iat: DateTime<Utc>  // Optional. Issued at
-    iss: String         // Optional. Issuer
-    nbf: DateTime<Utc>  // Optional. Not Before
-    sub: String,        // Optional. Subject (whom token refers to)
+    aud: String,         // Optional. Audience
+    exp: usize,          // Required (validate_exp defaults to true in validation). Expiration time
+    iat: usize,          // Optional. Issued at
+    iss: String,         // Optional. Issuer
+    nbf: usize,          // Optional. Not Before
+    sub: String,         // Optional. Subject (whom token refers to)
 }
 ```