Regex whitelist example in sample config

Users looking to the sample config to discover possible options may
not notice that its possible to whitelist by regex. This commit adds
an example to aid discovery.

Related: conjurinc/ops#246

gitleaks.toml

@@ -180,6 +180,9 @@ tags = ["key", "twilio"]
 files = [
   "(.*?)(jpg|gif|doc|pdf|bin)$"
 ]
+#regexes = [
+#  "AKIAIOSFODNN7EXAMPLE"
+#]
 #commits = [
 #  "whitelisted-commit1",
 #  "whitelisted-commit2",