Docs: Update repositories.asciidoc

1. Enable the repository using "add-apt-repository" to avoid this error "No command 'deb' found".
2. Adding "sudo" to update and install command.

Closes #8691

docs/reference/setup/repositories.asciidoc

@@ -27,14 +27,14 @@ Add the following to your /etc/apt/sources.list to enable the repository
 
 ["source","sh",subs="attributes,callouts"]
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-deb http://packages.elasticsearch.org/elasticsearch/{branch}/debian stable main
+sudo add-apt-repository "deb http://packages.elasticsearch.org/elasticsearch/{branch}/debian stable main"
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 Run apt-get update and the repository is ready for use. You can install it with :
 
 [source,sh]
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-apt-get update && apt-get install elasticsearch
+sudo apt-get update && sudo apt-get install elasticsearch
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 Configure Elasticsearch to automatically start during bootup :