typescript-fetch: fix missing comma in multiple imports (#3881)

Author: rcarton

modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

@@ -13,7 +13,7 @@ import {
 {{#discriminator}}
 import {
 {{#discriminator.mappedModels}}
-     {{modelName}}FromJSONTyped
+     {{modelName}}FromJSONTyped{{^-last}},{{/-last}}
 {{/discriminator.mappedModels}}
 } from './';