cp.c: adjust a comment to make it consistent with documentation

'-a' is equivalent to -dpR, so s/-dpPR/-dpR/;  the -P is redundant,
since -d already implies -P, and both --help and "info cp" say -dpR:
Corresponding doc changes: 080ac77850
b7c2db6766.

src/cp.c

@@ -920,7 +920,7 @@ main (int argc, char **argv)
 				     sparse_type_string, sparse_type);
 	  break;
 
-	case 'a':		/* Like -dpPR. */
+	case 'a':		/* Like -dpR. */
 	  x.dereference = DEREF_NEVER;
 	  x.preserve_links = true;
 	  x.preserve_ownership = true;