Help: Fix versionadded for VS 15 2017 instance selection

Support for `CMAKE_GENERATOR_INSTANCE` was added in CMake 3.11, but the
possibility was mentioned in a comment in older versions, so the wrong
versionadded value was used in commit c43e845d09 (Help: Add `..
versionadded` directives to generator docs, 2020-11-11,
v3.20.0-rc1~476^2).