[class.path] escape backslash in character literal

Fixes #731

Author: timsong-cpp

source/iostreams.tex

@@ -10646,7 +10646,7 @@
 \tcode{preferred_separator} is the slash character (\tcode{'/'}).
 For Windows based operating systems,
 \tcode{value_type} is \tcode{wchar_t} and
-\tcode{preferred_separator} is the backslash character (\tcode{L'\textbackslash'}).
+\tcode{preferred_separator} is the backslash character (\tcode{L'\textbackslash\textbackslash'}).
 \exitexample
 
 \rSec3[path.generic]{Generic pathname format}