docs: Fix invalid figure options in VTKFileFormats.md

This commit addresses the following warnings like the following introduced
in 056f0627 (vtkhdf: document transient data)

/path/to/VTK/Documentation/docs/design_documents/VTKFileFormats.md:1085:
WARNING: 'figure': Invalid options format (not a dict) [myst.directive_parse]

Documentation/docs/design_documents/VTKFileFormats.md

@@ -1083,9 +1083,9 @@ using the following formulas:
 
 
 ```{figure} poly_data_hdf_schema.png
----
-width=640
----
+:width: 640px
+:align: center
+
 Figure 8. - Poly Data VTKHDF File Format
 ```
 
@@ -1159,9 +1159,9 @@ absence of a data set, the appropriate geometry offsetting for the time step is
 place.
 
 ```{figure} transient_hdf_schema.png
----
-width=640
----
+:width: 640px
+:align: center
+
 Figure 10. - Transient Data VTKHDF File Format
 ```