Fix broken build

`server.session-timeout` was deprecated in 1.3 but still used in a sample
that broke once the support for it was removed. This commit fix the build
by using the expected property.

spring-boot-samples/spring-boot-sample-session-redis/src/main/resources/application.properties

-server.session-timeout: 5
\ No newline at end of file
+server.session.timeout=5
\ No newline at end of file