Docs: specify the units for DPX scanned size (AcademySoftwareFoundation#4694)

From the PDF:
https://pub.smpte.org/doc/st268/20030505-pub/st0268-2003.pdf

3.31 X scanned size: Defines the horizontal size of the original scanned
optical image in millimeters. [42.1]
3.32 Y scanned size: Defines the vertical size of the original scanned
optical image in millimeters. [42.2]

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This is a small PR that simply clarifies the units in the documentation.

Signed-off-by: Campbell Barton <[email protected]>

Author: ideasman42

src/doc/builtinplugins.rst

@@ -425,10 +425,10 @@ the `set_ioproxy()` methods.
      - reference high quantity
    * - ``dpx:XScannedSize``
      - float
-     - X scanned size
+     - X scanned size in millimeters
    * - ``dpx:YScannedSize``
      - float
-     - Y scanned size
+     - Y scanned size in millimeters
    * - ``dpx:FramePosition``
      - int
      - frame position in sequence