Changed shared credentials to use credential chain (aws-samples#114)

Co-authored-by: Marcos Lopez <lmarcosi@amazon.com>

Author: lmarcosi

ruby/ruby-pg/lib/hello_dsql.rb

@@ -4,7 +4,7 @@
 def example()
   cluster_endpoint = 'huabttnonxz4oajpjn53txpdue.dsql-gamma.us-east-1.on.aws'
   region = 'us-east-1'
-  credentials = Aws::SharedCredentials.new()
+  credentials = Aws::CredentialProviderChain.new.resolve
 
   begin
       token_generator = Aws::DSQL::AuthTokenGenerator.new({