fix(config-examples): fix bad configs

prior to this commit, the provided configs are not type-checked

rel/config/examples/listeners.ssl.conf.example

@@ -72,7 +72,7 @@ listeners.ssl.my_ssl_listener_name {
         ocsp {
              enable_ocsp_stapling = false
              responder_url = "http://ocsp.example.com"
-             issuer_pem = true
+             issuer_pem = "${EMQX_ETC_DIR}/certs/ocsp-issuer-cert.pem"
              refresh_http_timeout = 15s
              refresh_interval = 5m
         }

rel/config/examples/node.conf.example

@@ -31,7 +31,7 @@ node {
     ## Path to the persistent data directory.
     ## This config is pre-filled when the EMQX distribution package is built.
     ## You are advised to use the default value.
-    #data_dir = "data" # when running a zip package or in docker container
+    data_dir = "data" # when running a zip package or in docker container
     #data_dir = "/var/lib/emqx" # when installed from deb/rpm packages
 
     ## Type: Periodic garbage collection interval

rel/config/examples/plugin.conf.example

@@ -1,5 +1,5 @@
 ## Plugin management
-plugin {
+plugins {
     ## Plugins declaration
     ## Note: The plugins are started in the defined order
     states = [