Fix build. Refs symfony#12959

Author: OskarStark

.doctor-rst.yaml

@@ -75,6 +75,7 @@ whitelist:
         - '.. versionadded:: 1.11' # MakerBundle
         - '.. versionadded:: 1.3' # MakerBundle
         - '.. versionadded:: 1.8' # MakerBundle
+        - '.. versionadded:: 1.6' # Flex in setup/upgrade_minor.rst
         - '0 => 123' # assertion for var_dumper - components/var_dumper.rst
         - '1 => "foo"' # assertion for var_dumper - components/var_dumper.rst
         - '$var .= "Because of this `\xE9` octet (\\xE9),\n";'