Sprinkles: Fix examples in readme (#155)

Author: samrobbins85

packages/sprinkles/README.md

@@ -21,8 +21,8 @@ export const className = atoms({
     desktop: 'row'
   },
   background: {
-    lightMode: 'blue50',
-    darkMode: 'gray700'
+    lightMode: 'blue-50',
+    darkMode: 'gray-700'
   }
 });
 ```
@@ -127,6 +127,9 @@ const colors = {
   'blue-50': '#eff6ff',
   'blue-100': '#dbeafe',
   'blue-200': '#bfdbfe',
+  'gray-700': '#374151',
+  'gray-800': '#1f2937',
+  'gray-900': '#111827',
   // etc.
 };
 
@@ -169,8 +172,8 @@ export const container = atoms({
     desktop: 'row',
   },
   background: {
-    lightMode: 'blue50',
-    darkMode: 'gray700',
+    lightMode: 'blue-50',
+    darkMode: 'gray-700',
   }
 });
 ```