Update library/Zend/Mail/Transport/Sendmail.php

change key for additional_parameters argument

fix: 
sendmail error
sendmail: illegal option -- r
sendmail: usage: sendmail [ -t ] [ -fsender ] [ -Fname ] [ -bp ] [ -bs ] [ arg ... ]
Merge branch 'master' of git://github.com/craz/zf2-1 into hotfix/3261
[#3261] use -f switch instead of -r

- "-r" is evidently a deprecated/obsolete synonym for "-f", and not
  present on current versions of sendmail (though still present on many
  clones, including postfix, exim, and qmail)
  - updated tests to refer to "-f" instead of "-r"
Merge branch 'hotfix/3261'

Close #3261