Update README example for accessing headers

Using event.headers['Some-Header'] throws an error when the header is
not set. So it is better to use event.headers.get('Some-Header')
instead.

Signed-off-by: Mehdi Yedes <mehdi.yedes@gmail.com>

README.md

@@ -238,7 +238,7 @@ def handle(event, context):
     return {
         "statusCode": 200,
         "body": {
-            "content-type-received": event.headers['Content-Type']
+            "content-type-received": event.headers.get('Content-Type')
         }
     }
 ```