LibWeb: Align mfrac Padding with Updated MathML Core Spec

This aligns with the transition from the MathML Core Working Draft
(27 November 2023) to the Editor's Draft (26 November 2024).

Libraries/LibWeb/MathML/Default.css

@@ -71,8 +71,7 @@ mtd {
 
 /* Fractions */
 mfrac {
-  padding-inline-start: 1px;
-  padding-inline-end: 1px;
+  padding-inline: 1px;
 }
 mfrac > * {
   math-depth: auto-add;