docs/generate: remove uses of pkg/errors

While there may be reasons to keep pkg/errors in production code,
we don't need them for this generator code.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/generate/generate.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"errors"
 	"log"
 	"os"
 
@@ -12,7 +13,6 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/cli/command/commands"
-	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
 )
@@ -64,7 +64,7 @@ func gen(opts *options) error {
 				return err
 			}
 		default:
-			return errors.Errorf("unknown format %q", format)
+			return errors.New("unknown format: " + format)
 		}
 	}