Fix Puppet::HTTP::Client example with system store

The evolution of that part of the code has seen multiple iterations, and
the doc does not reflect the current usage so fix it.

While here, also fix the wrong variable name typo.

docs/http.md

@@ -260,7 +260,7 @@ possible to additionally trust the system store when making HTTPS requests:
 
 ```ruby
 client = Puppet::HTTP::Client.new
-response = http.get("https://artifactory.example.com/java.tar.gz", trust_system_store: true)
+response = client.get("https://artifactory.example.com/java.tar.gz", options: { include_system_store: true })
 response.read_body do |data|
   puts "Read #{data.bytes}"
 end