Command line to add package is incorrect. (#6558) (#6591)

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

Correct command is --prerelease

(cherry picked from commit ebde26a)

Co-authored-by: Robert <107894728+ivytechone@users.noreply.github.com>

Author: stevejgordon

docs/install.asciidoc

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