Fix a documentation typo.

docs/_data/links.json

@@ -54,7 +54,7 @@
     },
     "overview-variables": {
       "name": "Variables",
-      "subtitle": "See how Bulma uses <strong>Sass variables</strong> to allows easy customization",
+      "subtitle": "See how Bulma uses <strong>Sass variables</strong> to allow easy customization",
       "color": "grey",
       "icon": "cogs",
       "path": "/documentation/overview/variables"