Use two spaces instead of backslash to indicate hard break in markdown

Signed-off-by: Julius Härtl <jus@bitgrid.net>

src/extensions/Markdown.js

@@ -79,6 +79,7 @@ const createMarkdownSerializer = ({ nodes, marks }) => {
 		),
 		serialize(content, options) {
 			return this.serializer.serialize(content, { ...options, tightLists: true })
+				.split('\\\n').join('  \n')
 				.split('\\[').join('[')
 				.split('\\]').join(']')
 		},