Remove unneeded backticks (#43256)

docs/reference/modules/cluster/allocation_awareness.asciidoc

@@ -46,7 +46,7 @@ You can also set custom attributes when you start a node:
 +
 [source,sh]
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-`./bin/elasticsearch -Enode.attr.rack_id=rack_one`
+./bin/elasticsearch -Enode.attr.rack_id=rack_one
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 . Tell {es} to take one or more awareness attributes into account when