fix xref

Author: hauner

docs/modules/ROOT/pages/processor/requestbody.adoc

@@ -81,7 +81,7 @@ The file upload in OpenAPI is described like this:
                 format: binary
 ----
 
-(See also xref:howto/file_upload.adoc[OpenAPI - describing a file upload endpoint])
+(See also xref:oap:openapi:file_upload.adoc[OpenAPI - describing a file upload endpoint])
 
 The `schema` must be of type `object` defining a property for each part in the multipart body.