Update securing-your-webhooks.md (#1692)

The request header is `HTTP_X_HUB_SIGNATURE_256` not `HTTP_X_HUB_SIGNATURE_2`.

Author: usiegl00

content/developers/webhooks-and-events/securing-your-webhooks.md

@@ -72,7 +72,7 @@ end
 {% if currentVersion == "free-pro-team@latest" or currentVersion ver_gt "enterprise-server@2.22" or currentVersion == "github-ae@latest" %}
 def verify_signature(payload_body)
   signature = 'sha256=' + OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), ENV['SECRET_TOKEN'], payload_body)
-  return halt 500, "Signatures didn't match!" unless Rack::Utils.secure_compare(signature, request.env['HTTP_X_HUB_SIGNATURE_2'])
+  return halt 500, "Signatures didn't match!" unless Rack::Utils.secure_compare(signature, request.env['HTTP_X_HUB_SIGNATURE_256'])
 end{% elsif currentVersion ver_lt "enterprise-server@2.23" %}
 def verify_signature(payload_body)
   signature = 'sha1=' + OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha1'), ENV['SECRET_TOKEN'], payload_body)