Escape backslashes in .properties examples of setting internal-proxies

In a properties file, a backslash is used as an escape character for
the line terminator sequence to allow values to be split across
multiple lines. When a backslash is used elsewhere they're stripped
out of the property's value.

This commit updates .properties-based examples for configuring
server.tomcat.internal-proxies to escape the backslahes so that they
are retained in the property's value at runtime.

See gh-1989

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -70,10 +70,10 @@ content into your application; rather pick only the properties that you need.
 	server.ssl.trust-store-type=
 	server.tomcat.access-log-pattern= # log pattern of the access log
 	server.tomcat.access-log-enabled=false # is access logging enabled
-	server.tomcat.internal-proxies=10\.\d{1,3}\.\d{1,3}\.\d{1,3}|\
-			192\.168\.\d{1,3}\.\d{1,3}|\
-			169\.254\.\d{1,3}\.\d{1,3}|\
-			127\.\d{1,3}\.\d{1,3}\.\d{1,3} # regular expression matching trusted IP addresses
+	server.tomcat.internal-proxies=10\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}|\\
+			192\\.168\\.\\d{1,3}\\.\\d{1,3}|\\
+			169\\.254\\.\\d{1,3}\\.\\d{1,3}|\\
+			127\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3} # regular expression matching trusted IP addresses
 	server.tomcat.protocol-header=x-forwarded-proto # front end proxy forward header
 	server.tomcat.port-header= # front end proxy port header
 	server.tomcat.remote-ip-header=x-forwarded-for

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

@@ -493,7 +493,7 @@ to `application.properties`, e.g.
 
 [indent=0]
 ----
-	server.tomcat.internal_proxies=192\.168\.\d{1,3}\.\d{1,3}
+	server.tomcat.internal_proxies=192\\.168\\.\\d{1,3}\\.\\d{1,3}
 ----
 
 Alternatively, you can take complete control of the configuration of the `RemoteIpValve`