Fix typo in Readme file

Author: nanego

README.md

@@ -1,7 +1,7 @@
 Redmine Multiprojects Issue plugin
 ======================
 
-This redmine plugin allow you to specify more than one project per issue.
+This redmine plugin allows you to specify more than one project per issue.
 
 Cross-projects issues appear in all concerned projects. They can be viewed by every users who have enough permission on at least one impacted project, but can be updated only by users who have enough permissions on the main project (the project used to create the issue).