Fix typo in regexp-opt example code

* doc/lispref/searching.texi (Regexp Functions):
Fix typo in example code (Bug#34596).
1 job for master in 49 minutes and 27 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1170
test-all

00:49:27

 
Name Stage Failure
failed
test-all Test
Makefile:313: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:288: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:962: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1