chore(conf): set enablexsrf to false by default

comment: https://github.com/mindoc-org/mindoc/pull/662#pullrequestreview-622656551

conf/app.conf.example

@@ -7,7 +7,7 @@ runmode = "${MINDOC_RUN_MODE||dev}"
 sessionon = true
 sessionname = mindoc_id
 copyrequestbody = true
-enablexsrf = "${MINDOC_ENABLE_XSRF||true}"
+enablexsrf = "${MINDOC_ENABLE_XSRF||false}"
 
 #系统完整URL(http://doc.iminho.me),如果该项不设置，会从请求头中获取地址。
 baseurl="${MINDOC_BASE_URL}"