Fix plugin docs for using custom config dir

This commit fixes an issue with the plugin docs incorrectly specifying
how to set a custom configuration directory. The correct way is to use
the environment variable CONF_DIR.

docs/plugins/plugin-script.asciidoc

@@ -165,12 +165,9 @@ can do this as follows:
 
 [source,sh]
 ---------------------
-sudo bin/elasticsearch-plugin -Epath.conf=/path/to/custom/config/dir install <plugin name>
+sudo CONF_DIR=/path/to/conf/dir bin/elasticsearch-plugin install <plugin name>
 ---------------------
 
-You can also set the `CONF_DIR` environment variable to the custom config
-directory path.
-
 [float]
 === Proxy settings