Fix Plausible script (#402)

- use endpoint plausible.js in place of index.js
- use ASCII quotes

closes #388

layouts/partials/analytics/plausible.html

@@ -1 +1 @@
-<script async defer data-domain=”{{ .Site.Params.plausibleAnalytics.domain }}” src=”https://{{ .Site.Params.plausibleAnalytics.serverURL | default "plausible.io" }}/js/index.js”></script> 
+<script async defer data-domain="{{ .Site.Params.plausibleAnalytics.domain }}" src="https://{{ .Site.Params.plausibleAnalytics.serverURL | default "plausible.io" }}/js/plausible.js"></script>