Fix grammar and try to make clearer how a fork works

doc/gitlab-basics/fork-project.md

 # How to fork a project
 
-A fork is a copy of an original repository that you can put in another namespace
-where you can experiment and apply changes that you can later decide if
-publishing or not, without affecting your original project.
+A fork is a copy of an original repository that you put in another namespace
+where you can experiment and apply changes that you can later decide whether or
+not to share, without affecting the original project.
 
 It takes just a few steps to fork a project in GitLab.