Increase contrast for dark fg.subtle (#307)

* Increase contrast for dark `fg.subtle`

* Create perfect-carrots-doubt.md

Author: simurai

.changeset/perfect-carrots-doubt.md

@@ -0,0 +1,5 @@
+---
+"@primer/primitives": patch
+---
+
+Increase contrast for dark `fg.subtle`

data/colors/vars/global_dark.ts

@@ -4,7 +4,7 @@ export default {
   fg: {
     default: get('scale.gray.1'),
     muted: get('scale.gray.3'),
-    subtle: get('scale.gray.5'),
+    subtle: get('scale.gray.4'),
     onEmphasis: get('scale.white')
   },
   canvas: {