readme: fix syntax of format-source link

8 jobs for topic/default/README-fix-syntax in 5 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed checks-py2 #112747

00:01:25

passed checks-py3 #112748

00:01:26

passed doc #112753

00:00:27

manual pytype #112746
allowed to fail manual
passed tests-py2-cext #112749

00:05:46

passed tests-py2-pure #112750

00:05:49

passed tests-py3-cext #112751

00:05:42

passed tests-py3-pure #112752

00:05:47