Add mandatory 'stored' option to generated column example

The current implementation (rails#41856) required `stored: true` to be passed when using `t.virtual()`
https://github.com/rails/rails/pull/41856/files#diff-5b1b920fc3bebbb1d4c144b302603e5003a1d9e5a493e3234bf764b9e26fbe30R68-R75

Author: choznerol

guides/source/active_record_postgresql.md

@@ -562,7 +562,7 @@ create_table :documents do |t|
   t.string :body
 
   t.virtual :textsearchable_index_col,
-            type: :tsvector, as: "to_tsvector('english', title || ' ' || body)"
+            type: :tsvector, as: "to_tsvector('english', title || ' ' || body)", stored: true
 end
 
 add_index :documents, :textsearchable_index_col, using: :gin, name: 'documents_idx'