Fix for MathJax Preview Initialization Issue

The wagtailMathPreviews object was being reinitialized on every preview setup due to inconsistent casing, causing previously initialized previews to be overwritten and resulting in errors during Update() calls.

Author: MadScrewdriver

src/wagtailmath/static/wagtailmath/js/wagtailmath.js

@@ -81,7 +81,7 @@ class Preview {
 
 
 function initMathJaxPreview(id) {
-  window.wagtailMathPreviews = window.WagtailMathPreviews || {};
+  window.wagtailMathPreviews = window.wagtailMathPreviews || {};
 
   window.wagtailMathPreviews[id] = new Preview(
     "MathPreview-" + id,