Update api doc to reflect the S3 default integrity change

[jelninos]

Hello !

It seems that the Content-MD5 header is no longer required for the "delete-objects" method:

#3066
#3076

But your documentation still says that this header is required:

https://docs.aws.amazon.com/aws-sdk-php/v3/api/api-s3-2006-03-01.html#deleteobjects

Content-MD5 request header
General purpose bucket - The Content-MD5 request header is required for all Multi-Object Delete requests. Amazon S3 uses the header value to ensure that your request body has not been altered in transit.

Could you update it ?

Thanks !