Should use << for string concatenation in Ruby

plugins/category_generator.rb

@@ -174,8 +174,8 @@ ERR
     # Returns string
     def date_to_html_string(date)
       result = '<span class="month">' + date.strftime('%b').upcase + '</span> '
-      result += date.strftime('<span class="day">%d</span> ')
+      result << date.strftime('<span class="day">%d</span> ')
-      result += date.strftime('<span class="year">%Y</span> ')
+      result << date.strftime('<span class="year">%Y</span> ')
       result
     end