Minor improvement to English grammar in error msg

Fixed some unusual phrasing I saw while running maven commands.
Goal is to leave things even better than I found them :)

src/main/java/org/apache/maven/plugins/install/InstallFileMojo.java

@@ -185,7 +185,7 @@ public void execute()
 
         if ( !file.exists() )
         {
-            String message = "The specified file '" + file.getPath() + "' not exists";
+            String message = "The specified file '" + file.getPath() + "' does not exist";
             getLog().error( message );
             throw new MojoFailureException( message );
         }