Minor fixes in howto documentation re:SSL config

Fixes a couple of typos in the documentation:
* `x-forwarded-protocol` is not the standard name for this header
* `require_https` is not an existing property, but rather `require_ssl`

Author: bclozel

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -474,7 +474,7 @@ most proxies, and if you only set one the other will be set automatically):
 [indent=0]
 ----
 	server.tomcat.remote_ip_header=x-forwarded-for
-	server.tomcat.protocol_header=x-forwarded-protocol
+	server.tomcat.protocol_header=x-forwarded-proto
 ----
 
 If your proxy uses different headers you can customize the valve's configuration by adding
@@ -1450,7 +1450,7 @@ by adding some entries to `application.properties`, e.g.
 
 Spring Security can also be configured to require a secure channel for all (or some
 requests). To switch that on in a Spring Boot application you just need to set
-`security.require_https` to `true` in `application.properties`.
+`security.require_ssl` to `true` in `application.properties`.