cmd/help: fix example in doc

Closes #468
cuelang/cue#468

GitOrigin-RevId: df8fd25f7dc2a3b908b871778d26f3a27936e01a
Change-Id: I29bc6f9a8686f3b763825061ab023bfde15705f5
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7102
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/cmd.go

@@ -119,7 +119,7 @@ the second to standard output (i.e. it echos it again).
 		}
 
 		// starts after echo
-		file.Append & {
+		append: file.Append & {
 			filename: var.file
 			contents: echo.stdout
 		}

cmd/cue/cmd/help.go

@@ -423,7 +423,7 @@ the second to standard output (i.e. it echos it again).
 		}
 
 		// starts after echo
-		file.Append & {
+		append: file.Append & {
 			filename: var.file
 			contents: echo.stdout
 		}

cmd/cue/cmd/testdata/script/help_cmd.txt

@@ -116,7 +116,7 @@ the second to standard output (i.e. it echos it again).
 		}
 
 		// starts after echo
-		file.Append & {
+		append: file.Append & {
 			filename: var.file
 			contents: echo.stdout
 		}