docs: 📝 "http get", add an example with more than one header (#9240)

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I was trying `-H [key-A val-A] -H [key-B val-B]` but thanks to @Dorumin
I discovered how it should be used.
This PR just adds an extra example to the help with multiple headers.
Feel free to edit it either to merge both header examples into one or
rename the key value used, but I think it would be nice to have a sample
as the multiple -H variant doesn't error out it wasn't obvious to me.

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One more help example in `help http get`

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crates/nu-command/src/network/http/get.rs

@@ -118,6 +118,11 @@ impl Command for SubCommand {
                 example: "http get -H [my-header-key my-header-value] https://www.example.com",
                 result: None,
             },
+            Example {
+                description: "Get content from example.com, with custom headers",
+                example: "http get -H [my-header-key-A my-header-value-A my-header-key-B my-header-value-B] https://www.example.com",
+                result: None,
+            },
         ]
     }
 }