cmd/completion: Replace deprecated github.com/spf13/cobra API

The cobra.ExactValidArgs() API [1] was deprecated in Cobra 1.6.0 [2] in
favour of combining different cobra.PositionalArgs.

Commit 00f47b2 already bumped the required
github.com/spf13/cobra version to 1.6.0, so adjust accordingly.

[1] https://pkg.go.dev/github.com/spf13/cobra#ExactValidArgs

[2] Cobra commit 70e53f62be77e03f
    spf13/cobra@70e53f62be77e03f
    spf13/cobra#1643
    spf13/cobra#745
    spf13/cobra#838

containers#1776

Author: debarshiray

src/cmd/completion.go

@@ -32,7 +32,7 @@ var completionCmd = &cobra.Command{
 	Hidden:                true,
 	DisableFlagsInUseLine: true,
 	ValidArgs:             []string{"bash", "fish", "zsh"},
-	Args:                  cobra.ExactValidArgs(1),
+	Args:                  cobra.MatchAll(cobra.ExactArgs(1), cobra.OnlyValidArgs),
 	RunE:                  completion,
 }