Unify "file-descriptor" to "file descriptor".
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 24830d2..bc4cf42 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -651,7 +651,7 @@
 .. function:: ttyname(fd)
 
    Return a string which specifies the terminal device associated with
-   file-descriptor *fd*.  If *fd* is not associated with a terminal device, an
+   file descriptor *fd*.  If *fd* is not associated with a terminal device, an
    exception is raised. Availability:Macintosh, Unix.