use plain char if no translation in troffchar (fix ", "" in wintext(1))

src/cmd/troff2html/troff2html.c

diff --git a/src/cmd/troff2html/troff2html.c b/src/cmd/troff2html/troff2html.c
index c0956c7..91145c0 100644
--- a/src/cmd/troff2html/troff2html.c
+++ b/src/cmd/troff2html/troff2html.c
@@ -705,7 +705,7 @@
 	for(i=0; troffchars[i].name!=nil; i++)
 		if(strcmp(s, troffchars[i].name) == 0)
 			return troffchars[i].value;
-	return "??";
+	return s;
 }
 
 void