Update "Custom Property" Page to new shortcode "vec-diagram"

Change content/specifications/vec/guidelines/key-concepts/custom-properties/index.md line 39 from:

These examples are intended as an extension to this specification: [Extensibility with Custom Properties]({{< relref "specifications/vec/v120/basic-datatypes/extensibility-with-custom-properties/index.md">}}).

to:

These examples are intended as an extension to this specification: {{< vec-diagram "basic-datatypes/extensibility-with-custom-properties" >}}.