Fix typo (#164)

Change-Id: I649d0505a4726abed6c95c37c8ebdbbdb471e041
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3860
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/doc/cmd/cue.md b/doc/cmd/cue.md
index 58677ba..e8b6d0c 100644
--- a/doc/cmd/cue.md
+++ b/doc/cmd/cue.md
@@ -468,7 +468,7 @@
 
   // starts after ask
   task echo: exec.Run({
-   cmd:    "echo Hello \(task.ask.response)!"]
+   cmd:    "echo Hello \(task.ask.response)!"
    stdout: string // capture stdout
   })
 
@@ -483,4 +483,4 @@
    contents: task.echo.stdout
   })
 }
-```
\ No newline at end of file
+```