Patch #1376914: traceback.format_exc() has no "file" argument

Doc/lib/libtraceback.tex

@@ -48,7 +48,7 @@ information in a thread-safe way instead of using the deprecated
 variables.)
 \end{funcdesc}
 
-\begin{funcdesc}{format_exc}{\optional{limit\optional{, file}}}
+\begin{funcdesc}{format_exc}{\optional{limit}}
 This is like \code{print_exc(\var{limit})} but returns a string
 instead of printing to a file.
 \versionadded{2.4}