Minor markup adjustments.

Doc/lib/libos.tex

@@ -300,8 +300,8 @@ function.
 Availability: Macintosh, \UNIX, Windows.
 
 \versionchanged[When specified, the \var{mode} argument must now start
-  with one of the letters \code{'r'}, \code{'w'}, or \code{'a'},
-  otherwise a \exception{ValueError} is raised. ]{2.3}
+  with one of the letters \character{r}, \character{w}, or \character{a},
+  otherwise a \exception{ValueError} is raised]{2.3}
 \end{funcdesc}
 
 \begin{funcdesc}{popen}{command\optional{, mode\optional{, bufsize}}}