[chore] fix type in confighttp readme (#9690)

This doc incorrectly uses the otlp exporter as an example of using
confighttp. That exporter doesn't use confighttp

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

config/confighttp/README.md

@@ -39,7 +39,7 @@ Example:
 
 ```yaml
 exporter:
-  otlp:
+  otlphttp:
     endpoint: otelcol2:55690
     auth:
       authenticator: some-authenticator-extension