writeline() --> writelines()

This closes SF bug #487147.

Doc/lib/libdifflib.tex

@@ -466,7 +466,7 @@ method:
   with newlines. Such sequences can be obtained from the
   \method{readlines()} method of file-like objects.  The delta generated
   also consists of newline-terminated strings, ready to be printed as-is
-  via the \method{writeline()} method of a file-like object.
+  via the \method{writelines()} method of a file-like object.
 \end{methoddesc}