repair string literal.
diff --git a/Doc/lib/liboptparse.tex b/Doc/lib/liboptparse.tex
index 317b88e..dd618c8 100644
--- a/Doc/lib/liboptparse.tex
+++ b/Doc/lib/liboptparse.tex
@@ -518,7 +518,7 @@
is then printed before the detailed option help.
If you don't supply a usage string, \module{optparse} uses a bland but sensible
-default: \code{usage: {\%}prog {[}options]"}, which is fine if your script
+default: \code{"usage: {\%}prog {[}options]"}, which is fine if your script
doesn't take any positional arguments.
\item {}