Add help text to show yaml export option

In #182, support for `cue export --out yaml` was added
but this wasn't reflected in the help text.

Closes #297
https://github.com/cuelang/cue/pull/297

GitOrigin-RevId: 352fb4a4e04111e64df0f54ae4a407d9215832e9
Change-Id: Iff3343aa4f47ed115ec1f41445af854cfda6a0d9
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5088
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cmd/cue/cmd/export.go b/cmd/cue/cmd/export.go
index 7957a29..385e482 100644
--- a/cmd/cue/cmd/export.go
+++ b/cmd/cue/cmd/export.go
@@ -78,10 +78,13 @@
 The following formats are recognized:
 
 json    output as JSON
-		Outputs any CUE value.
+               Outputs any CUE value.
 
 text    output as raw text
-        The evaluated value must be of type string.
+                The evaluated value must be of type string.
+
+yaml    output as YAML
+                Outputs any CUE value.
 `,
 
 		RunE: mkRunE(c, runExport),