[FIX]error string should not be capitalized

Signed-off-by: Scaat Feng <scaat.feng@gmail.com>

pkg/plugin/plugin.go

@@ -151,7 +151,7 @@ func (p *Plugin) PrepareCommand(extraArgs []string) (string, []string, error) {
 		parts = strings.Split(os.ExpandEnv(p.Metadata.Command), " ")
 	}
 	if len(parts) == 0 || parts[0] == "" {
-		return "", nil, fmt.Errorf("No plugin command is applicable")
+		return "", nil, fmt.Errorf("no plugin command is applicable")
 	}
 
 	main := parts[0]