Get doc to build (add missing backslash)

Doc/lib/libos.tex

@@ -595,7 +595,7 @@ These can be bit-wise OR'd together.
 Availability: Macintosh, \UNIX, Windows.
 \end{datadesc}
 
-begin{datadesc}{O_DSYNC}
+\begin{datadesc}{O_DSYNC}
 \dataline{O_RSYNC}
 \dataline{O_SYNC}
 \dataline{O_NDELAY}