[v6.d REVIEW] Fix up Proc.command

It returns an immutable List, not a mutable Array

Rakudo   fix up: rakudo/rakudo@925c113433
Propspec fix up: Raku/roast@d66c1a3349

Author: zoffixznet

doc/Type/Proc.pod6

@@ -177,9 +177,9 @@ through to the shell for parsing and execution. See L<C<IO::shell>|/type/IO#sub_
 
 =head2 method command
 
-    method command(Proc:D: --> Array:D)
+    method command(Proc:D: --> List:D)
 
-The command method is an accessor to an array containing the arguments that were
+The command method is an accessor to a list containing the arguments that were
 passed when the Proc object was executed via C<spawn> or C<shell> or C<run>.
 
 =head2 method pid