doc: Fix command example to run regression tests with PGOPTIONS

The documentation mentioned the use of log_checkpoints, that cannot be
used in this context.  This commit replaces log_checkpoints with
force_parallel_mode, a developer option useful to perform checks related
to parallelism.

Oversight in 854434c5.

Author: Haiying Tang
Discussion: https://postgr.es/m/OS0PR01MB6113954B883ACEB2DDC973F2FBE59@OS0PR01MB6113.jpnprd01.prod.outlook.com
Backpatch-through: 14

doc/src/sgml/regress.sgml

@@ -359,7 +359,7 @@ make check LANG=C ENCODING=EUC_JP
     set in the <varname>PGOPTIONS</varname> environment variable (for settings
     that allow this):
 <screen>
-make check PGOPTIONS="-c log_checkpoints=on -c work_mem=50MB"
+make check PGOPTIONS="-c force_parallel_mode=regress -c work_mem=50MB"
 </screen>
     When running against a temporary installation, custom settings can also be
     set by supplying a pre-written <filename>postgresql.conf</filename>: