Fix error when table_name is frozen (#517)

lib/annotate/annotate_models.rb

@@ -212,8 +212,8 @@ module AnnotateModels
       return indexes if indexes.any? || !klass.table_name_prefix
 
       # Try to search the table without prefix
-      table_name.to_s.slice!(klass.table_name_prefix)
-      klass.connection.indexes(table_name)
+      table_name_without_prefix = table_name.to_s.sub(klass.table_name_prefix, '')
+      klass.connection.indexes(table_name_without_prefix)
     end
 
     # Use the column information in an ActiveRecord class