Fix styleSheets manifest key in package.json of fixtures

spec/fixtures/packages/theme-with-incomplete-ui-variables/package.json

@@ -1,4 +1,4 @@
 {
   "theme": "ui",
-  "stylesheets": ["editor.less"]
+  "styleSheets": ["editor.less"]
 }

spec/fixtures/packages/theme-with-syntax-variables/package.json

@@ -1,4 +1,4 @@
 {
   "theme": "syntax",
-  "stylesheets": ["editor.less"]
+  "styleSheets": ["editor.less"]
 }