Oiiotool documentation refers to nonexisting command --then

[rasmusbonnedal]

OpenImageIO/src/doc/oiiotool.rst

Line 272 in 43b8aa9

* Conditionals : `--if` *condition* `--then` *commands...* `--else` *commands...* `--endif`

The Oiiotool documentation for --if refers to --then which does not exist.