updated Readme.md

Readme.md

@@ -118,9 +118,13 @@ The **default credentials** chain in this plugin is
 awsProfile := Some("default")
 
 s3credentials :=
-  new ProfileCredentialsProvider(awsProfile.value.orNull) |
-  new EnvironmentVariableCredentialsProvider() |
-  new InstanceProfileCredentialsProvider()
+  (awsProfile.value match {
+    case Some(profile) => new ProfileCredentialsProvider(profile)
+    case _ =>
+      new ProfileCredentialsProvider() |
+      new EnvironmentVariableCredentialsProvider() |
+      InstanceProfileCredentialsProvider.getInstance()
+  })
 ```
 
 * [`new ProfileCredentialsProvider(...)`](http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/auth/ProfileCredentialsProvider.html) which loads credentials for an AWS profile config file