Command line to add package is incorrect. (#6558)

dotnet add package Elastic.Clients.Elasticsearch -prerelease
Unrecognized command or argument '-prerelease'

Correct command is --prerelease

Author: ivytechone

docs/install.asciidoc

@@ -19,7 +19,7 @@ For SDK style projects, you can install the {es} client by running the following
 
 [source,text]
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-dotnet add package Elastic.Clients.Elasticsearch -prerelease
+dotnet add package Elastic.Clients.Elasticsearch --prerelease
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 This command adds a package reference to your project (csproj) file for the