[FAB-6780] Correct typo in statecouchdb_test.go

The last test in TestUtilityFunctions() has an incorrect comment.
It states "// ValidateKey should return nil for a valid key"
it should say "// ValidateKey should return error for an invalid key"

Change-Id: Ie7ccf954083d5f3eeb0f879664efb4afb5717b68
Signed-off-by: Chris Elder <chris.elder@us.ibm.com>

core/ledger/kvledger/txmgmt/statedb/statecouchdb/statecouchdb_test.go

@@ -171,7 +171,7 @@ func TestUtilityFunctions(t *testing.T) {
 	err = db.ValidateKey("testKey")
 	testutil.AssertNil(t, err)
 
-	// ValidateKey should return nil for a valid key
+	// ValidateKey should return an error for an invalid key
 	err = db.ValidateKey(string([]byte{0xff, 0xfe, 0xfd}))
 	testutil.AssertError(t, err, "ValidateKey should have thrown an error for an invalid utf-8 string")