Limits Riggs-class ships to one (#2577)

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## About The Pull Request
This ship is decently sized and supports a decently sized crew. Having
multiple diminishes that by spreading out players if multiple are
spawned around the same time.
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## Changelog

:cl:
tweak: Multiple Riggs-classes cannot be spawned by players anymore
/:cl:

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_maps/configs/independent_rigger.json

@@ -18,7 +18,7 @@
 	],
 	"map_path": "_maps/shuttles/independent/independent_rigger.dmm",
 	"roundstart": true,
-	"limit": 2,
+	"limit": 1,
 	"job_slots": {
 		"Captain": {
 			"outfit": "/datum/outfit/job/captain/western",