Use underscores for em text in Rich editor

This fixes an issue where mixing strong and em text caused the text
to not render correctly as a post (e.g. *This is **bold text**.*
would fail).

prose/markdownSerializer.js

@@ -92,8 +92,8 @@ export const writeFreelyMarkdownSerializer = new MarkdownSerializer(
   },
   {
     em: {
-      open: "*",
-      close: "*",
+      open: "_",
+      close: "_",
       mixable: true,
       expelEnclosingWhitespace: true,
     },