Fix Plural-Forms of i18n for english language

[alexbarcelo]

It still receives warnings:

$> msgfmt -c .../djangocms-bootstrap4/djangocms_bootstrap4/locale/en/LC_MESSAGES

msgfmt: ./django.po: warning: PO file header fuzzy                                             
                     warning: older versions of msgfmt will give an error on this                          
./django.po:8: warning: header field 'Project-Id-Version' still has the initial default value
./django.po:8: warning: header field 'PO-Revision-Date' still has the initial default value    
./django.po:8: warning: header field 'Last-Translator' still has the initial default value
./django.po:8: warning: header field 'Language-Team' still has the initial default value                                      
./django.po:8: warning: header field 'Language' still has the initial default value                                        

But the critical error (see #58) should be fixed with this.

[codecov-io]

Codecov Report

Merging #60 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #60   +/-   ##
=======================================
  Coverage   66.51%   66.51%           
=======================================
  Files          63       63           
  Lines        1290     1290           
  Branches       98       98           
=======================================
  Hits          858      858           
  Misses        424      424           
  Partials        8        8

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[filwaitman]

Hi @alexbarcelo! Thanks for the PR!

This looks good to me. @kinkerl can you take a look as well?