better example of user defined supporting file (#18512)

Author: martin-mfg

bin/configs/java-okhttp-user-defined-templates.yaml

@@ -6,7 +6,7 @@ templateDir: modules/openapi-generator/src/main/resources/Java
 additionalProperties:
   hideGenerationTimestamp: "true"
 files:
-  README.md:
+  README.mustache:
     destinationFilename: info.md
   api_doc.mustache:
     templateType: APIDocs

samples/client/echo_api/java/okhttp-gson-user-defined-templates/.openapi-generator/FILES

@@ -1,7 +1,6 @@
 .github/workflows/maven.yml
 .gitignore
 .travis.yml
-README.md
 api/openapi.yaml
 build.gradle
 build.sbt