Update repositories.asciidoc

Added apt-get update to the commands for clarity.

Closes #7192

docs/reference/setup/repositories.asciidoc

@@ -27,7 +27,7 @@ Run apt-get update and the repository is ready for use. You can install it with
 
 [source,sh]
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-apt-get install elasticsearch
+apt-get update && apt-get install elasticsearch
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