Fix typo.
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 6a44029..b93ce77 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -1080,7 +1080,7 @@
of its subdirectories (directories are generated bottom up).
When \var{topdown} is true, the caller can modify the \var{dirnames} list
-in-place (perhaps unsing \keyword{del} or slice assignment), and
+in-place (perhaps using \keyword{del} or slice assignment), and
\function{walk()} will only recurse into the subdirectories whose names
remain in \var{dirnames}; this can be used to prune the search,
impose a specific order of visiting, or even to inform \function{walk()}