Multiple 'To' addresses in Report Settings causes malformed To: header in outbound email

The help text for the 'Email Delivery Settings' section in the 'Report Settings' includes the statement, 'Multiple email addresses should be separated by commas.' Entering two or more addresses separated by commas results in a badly formatted To: header in the outbound email.

The CRM_Utils_Mail::send function that actually sends the report expects the 'To' email to be a single address.

Attached patch changes the help text to make this more clear.