diff --git a/doc_src/cmds/isatty.rst b/doc_src/cmds/isatty.rst
index 879f0d027..857a09c5e 100644
--- a/doc_src/cmds/isatty.rst
+++ b/doc_src/cmds/isatty.rst
@@ -8,7 +8,7 @@ Synopsis
 
 .. synopsis::
 
-    isatty [FILE DESCRIPTOR]
+    isatty [FILE_DESCRIPTOR]
 
 Description
 -----------