fix(doc): IAM Actions must be arrays

Ref issue aws-samples#20. Users observed AccessDeniedException when setting IAM
policy "Action": "bedrock:*". Per the IAM grammar, it looks like a
single string is only valid for "*": Other actions must be enclosed
in an array.

https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_grammar.html

README.md

@@ -51,7 +51,7 @@ To grant Bedrock access to your identity, you can:
         {
             "Sid": "BedrockFullAccess",
             "Effect": "Allow",
-            "Action": "bedrock:*",
+            "Action": ["bedrock:*"],
             "Resource": "*"
         }
     ]