fix(markdown.scss): replace :first-of-type with :first-child for consistent styling of first elements

src/renderer/src/assets/styles/markdown.scss

@@ -8,16 +8,16 @@
     margin-bottom: 5px;
   }
 
-  p:first-of-type {
+  p:first-child {
     margin-top: 0;
   }
 
-  h1:first-of-type,
-  h2:first-of-type,
-  h3:first-of-type,
-  h4:first-of-type,
-  h5:first-of-type,
-  h6:first-of-type {
+  h1:first-child,
+  h2:first-child,
+  h3:first-child,
+  h4:first-child,
+  h5:first-child,
+  h6:first-child {
     margin-top: 0;
   }