Add / consistently in logback base.xml

Update logback `base.xml` so that both `LOG_PATH` and `LOG_TEMP` can
be specified without a trailing slash.

Fixes spring-projectsgh-6423

spring-boot/src/main/resources/org/springframework/boot/logging/logback/base.xml

@@ -6,7 +6,7 @@ Base logback configuration provided for compatibility with Spring Boot 1.1
 
 <included>
 	<include resource="org/springframework/boot/logging/logback/defaults.xml" />
-	<property name="LOG_FILE" value="${LOG_FILE:-${LOG_PATH:-${LOG_TEMP:-${java.io.tmpdir:-/tmp}}/}spring.log}"/>
+	<property name="LOG_FILE" value="${LOG_FILE:-${LOG_PATH:-${LOG_TEMP:-${java.io.tmpdir:-/tmp}}}/spring.log}"/>
 	<include resource="org/springframework/boot/logging/logback/console-appender.xml" />
 	<include resource="org/springframework/boot/logging/logback/file-appender.xml" />
 	<root level="INFO">