Fix literal backslash syntax

In CommonMark, there are no backslash escapes in code spans, so only two backslashes in the source document are necessary to produce two backslashes in the output document.

docs/markdown/Syntax.md

@@ -84,7 +84,7 @@ single quote do it like this:
 single quote = 'contains a \' character'
 ```
 
-Similarly `\n` gets converted to a newline and `\\\\` to a single
+Similarly `\n` gets converted to a newline and `\\` to a single
 backslash.
 
 #### String concatenation