cmd/cue/cmd: correct documentation for -d flag

Fixes #751

Change-Id: I3bb1005c33e50dbdcc3b487ee1d1d5f51354f816
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8782
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

cmd/cue/cmd/vet.go

@@ -46,7 +46,7 @@ contain multiple objects (such as using --- in YAML), they will all be verified
 individually.
 
 By default, each file is checked against the root of the loaded CUE files.
-The -e can be used to only verify files against the result of an expression
+The -d can be used to only verify files against the result of an expression
 evaluated within the CUE files. This can be useful if the CUE files contain
 a set of definitions to pick from.
 
@@ -56,7 +56,7 @@ Examples:
   cue vet foo.yaml foo.cue
 
   # Check files against a particular expression
-  cue vet translations/*.yaml foo.cue -e Translation
+  cue vet translations/*.yaml foo.cue -d '#Translation'
 
 If more than one expression is given, all must match all values.
 `