Use Sqlite3 adapter in examples

Two bits of example code use sqlite as an adapter, which doesn't exist.
Using the code verbatim will raise a LoadError exception:

    ActiveRecord::Base.establish_connection(
      "adapter"  => "sqlite",
      "database" => "db.sqlite"
    )
    # => LoadError: Could not load 'active_record/connection_adapters/sqlite_adapter'...

Considering this is code a lot of people new to Rails might be running,
it's especially confusing.

Closes #14367 [ci skip]

activerecord/lib/active_record/connection_handling.rb

@@ -18,14 +18,14 @@ module ConnectionHandling
     # Example for SQLite database:
     #
     #   ActiveRecord::Base.establish_connection(
-    #     adapter:  "sqlite",
+    #     adapter:  "sqlite3",
     #     database: "path/to/dbfile"
     #   )
     #
     # Also accepts keys as strings (for parsing from YAML for example):
     #
     #   ActiveRecord::Base.establish_connection(
-    #     "adapter"  => "sqlite",
+    #     "adapter"  => "sqlite3",
     #     "database" => "path/to/dbfile"
     #   )
     #