doc: Replace reference to ERRCODE_RAISE_EXCEPTION by "raise_exception"

This part of the documentation refers to exceptions as handled by PL/pgSQL, and using the internal error code is confusing. Per thinko in 66bde49. Reported-by: Euler Taveira, Bruce Momjian Discussion: https://postgr.es/m/ZUEUnLevXyW7DlCs@momjian.us Backpatch-through: 11

Author: michaelpq

doc/src/sgml/plpgsql.sgml

@@ -3942,7 +3942,7 @@ RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;
  <para>
  If no condition name nor SQLSTATE is specified in a
  <command>RAISE EXCEPTION</command> command, the default is to use
- <literal>ERRCODE_RAISE_EXCEPTION</literal> (<literal>P0001</literal>).
+ <literal>raise_exception</literal> (<literal>P0001</literal>).
  If no message text is specified, the default is to use the condition
  name or SQLSTATE as message text.
  </para>