Updated docstring

[radarhere]

Before: https://pillow.readthedocs.io/en/latest/reference/plugins.html#PIL.TiffImagePlugin.ImageFileDirectory_v2

After: https://pillow--5708.org.readthedocs.build/en/5708/reference/plugins.html#PIL.TiffImagePlugin.ImageFileDirectory_v2

[hugovk]

The docs step failure in the 3.9 build looks relevant:

[...]/PIL/TiffImagePlugin.py:docstring of PIL.TiffImagePlugin.ImageFileDirectory_v2:26: WARNING: py:attr reference target not found: PIL.TiffTags.TYPES

And TYPES is shown without linking at https://pillow--5708.org.readthedocs.build/en/5708/reference/plugins.html#PIL.TiffImagePlugin.ImageFileDirectory_v2

[hugovk]

Thanks!