Use `--extended-regexp` in lint-doc for compatibility with Darwin grep

scripts/lint-doc.sh

@@ -3,7 +3,7 @@
 cd "$(dirname "$0")/.."
 
 # Use long options (e.g. --header instead of -H) for curl examples in documentation.
-grep --perl-regexp --recursive --color=auto 'curl (.+ )?-[^- ].*' doc/
+grep --extended-regexp --recursive --color=auto 'curl (.+ )?-[^- ].*' doc/
 if [ $? == 0 ]
 then
   echo '✖ ERROR: Short options should not be used in documentation!' >&2