Update dic_tags.rst

  SwiftMailer plugin works only if its service defined with tag 'swiftmailer.default.plugin' and does not work with tag 'swiftmailer.plugin' specified in documentation
Update dic_tags.rst
Update dic_tags.rst
Update dic_tags.rst
Merge pull request #3201 from mgrinko/patch-1

DI-tags documentation should contain 'swiftmailer.default.plugin' tag instead of 'swiftmailer.plugin' to work