Update theme.json

Rubik font was default inadvertently. This fixes the theme to use the correct font.

mpho/theme.json

@@ -557,7 +557,7 @@
 			},
 			"heading": {
 				"typography": {
-					"fontFamily": "var(--wp--preset--font-family--rubik)",
+					"fontFamily": "var(--wp--preset--font-family--system-font)",
 					"fontWeight": "400",
 					"lineHeight": "1.125"
 				}
@@ -600,4 +600,4 @@
 	],
 	"version": 2,
 	"$schema": "https://schemas.wp.org/trunk/theme.json"
-}
+}