Changed JAVA_OPTS to ES_JAVA_OPTS in plugin docs

docs/plugins/plugin-script.asciidoc

@@ -207,7 +207,7 @@ options can be set on the command line:
 sudo bin/elasticsearch-plugin install mobz/elasticsearch-head -DproxyHost=host_name -DproxyPort=port_number
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-On Windows, they need to be added to the `JAVA_OPTS` environment variable:
+On Windows, they need to be added to the `ES_JAVA_OPTS` environment variable:
 
 [source,shell]
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