docs: Fix metadata/headers example on confighttp README [chore] (open…

…-telemetry#11317)

#### Description

Documentation fix: The example for using attributesprocessor to add the
value of an incoming HTTP header missed the `metadata.` prefix and the
lowercasing of the header name.

Fix the example in config/confighttp/README.md.

config/confighttp/README.md

@@ -111,7 +111,7 @@ processors:
   attributes:
     actions:
       - key: http.client_ip
-        from_context: X-Forwarded-For
+        from_context: metadata.x-forwarded-for
         action: upsert
 ```