Modify DpDemo to use 0 for default priority

F´ Version	4.3
Affected Component	Ref/DpDemo

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Feature Description

Change the Dp command to use zero to indicate using default priority.

Rationale

At JPL, the convention is that if a command has a priority argument, a value of zero means that the default priority should be used. The current example never has a chance to use the default priority specified in the FPP, and this would illustrate one way to do it.