modified regex for finding table_name from a multiline sql query in postgresql

In Postgresql, while inserting a sql query we find the table name using `RegExp` defined in `#extract_table_ref_from_insert_sql`.

```ruby
Before:
sql_query = <<-SQL
    insert into articles(
      number)
    values(
      5152
    )
    SQL
 extract_table_ref_from_insert_sql(sql_query) # => nil


After:
sql_query = <<-SQL
    insert into articles(
      number)
    values(
      5152
    )
    SQL
 extract_table_ref_from_insert_sql(sql_query) # => articles

```

Fixes issue #13378