FindTIFF: Fix name of upstream TIFF cmake package

Since commit 6a3059e66f (FindTIFF: bridge `tiff-config` into
FindTIFF-compatible interface, 2023-09-14, v3.28.0-rc1~87^2)
we try to find the upstream TIFF cmake package.  However, it
is called `TiffConfig.cmake`, not `tiff-config.cmake`, so we
need to match the capitalization of the package name.