docs: Fix misstatement about oiiotool --if (AcademySoftwareFoundation#5102)

There is no `--then`, you just say `--if condition commands...`

Fixes AcademySoftwareFoundation#5068

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/doc/oiiotool.rst

@@ -269,7 +269,7 @@ Control flow
 Scriptability is provided by the use of control flow statements.
 The usual programming constructs are supported:
 
-* Conditionals : `--if` *condition* `--then` *commands...* `--else` *commands...* `--endif`
+* Conditionals : `--if` *condition* *commands...* `--else` *commands...* `--endif`
 
 * General looping: `--while` *condition* *commands...* `--endwhile`