[Ada] Tune comment about expansion of array equality

[Pierre-Marie de Rodat]

Cleanup related to expansion of dispatching equality for GNATprove.
Semantics is unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch4.adb (Arr_Attr): Refine type of the parameter from Int
	to Pos; refine name of the parameter from Num to Dim; fix
	reference to "Expr" in comment.
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