Update app.py

app.py

@@ -90,7 +90,7 @@ if __name__ == "__main__":
     with open(config['config-path'], "w") as outfile:
         config['buildVersion'] = VERSION
         json.dump(config, outfile)
-    with open(config['openapi-yaml-path'], "r+w") as outfile:
+    with open(config['openapi-yaml-path'], "r+") as outfile:
         info = outfile.read()
         outfile.seek(0)
         outfile.write(info.replace('$VERSION_VARIABLE$', VERSION))