Update plugins.adoc (neo4j#1472)

Copied from neo4j#1438

Author: renetapopova

modules/ROOT/pages/kubernetes/plugins.adoc

@@ -94,7 +94,7 @@ Building a Docker container image that is based on the official Neo4j Docker ima
 [source, Dockerfile, subs=attributes]
 ----
 ARG  NEO4J_VERSION
-FROM neo4j:\{NEO4J_VERSION}
+FROM neo4j:$\{NEO4J_VERSION}
 
 # copy my-plugins into the Docker image
 COPY my-plugins/ /var/lib/neo4j/plugins