Update README to match the new syntax (#630)

This patch from 2 weeks ago changed the config syntax:

c0f382b

however, no change was made to the README. This fixes it, already confused few people I assume :)

Cheers

Co-authored-by: Marcin Romaszewicz <47459980+deepmap-marcinr@users.noreply.github.com>

README.md

@@ -678,16 +678,18 @@ file via the `--config` option. Please see the test under
 for an example. The structure of the file is as follows:
 
 ```yaml
-output:
-  externalref.gen.go
 package: externalref
 generate:
   models: true
+  embedded-spec: true
 output-options:
   skip-prune: true
 import-mapping:
   ./packageA/spec.yaml: github.com/deepmap/oapi-codegen/internal/test/externalref/packageA
   ./packageB/spec.yaml: github.com/deepmap/oapi-codegen/internal/test/externalref/packageB
+output: externalref.gen.go
+output-options:
+  skip-prune: true
 ```
 
 Have a look at [`cmd/oapi-codegen/oapi-codegen.go`](https://github.com/deepmap/oapi-codegen/blob/master/cmd/oapi-codegen/oapi-codegen.go#L48)