Typo.

Doc/lib/libdoctest.tex

@@ -20,7 +20,7 @@ use doctest:
 \item To perform regression testing by verifying that interactive
       examples from a test file or a test object work as expected.
 \item To write tutorial documentation for a package, liberally
-      illustrated with input-ouput examples.  Depending on whether
+      illustrated with input-output examples.  Depending on whether
       the examples or the expository text are emphasized, this has
       the flavor of "literate testing" or "executable documentation".
 \end{itemize}