Remove placeholder line in WritingCommandPlugin.md (#9440)

Remove placeholder line in WritingCommandPlugin.md.

### Motivation:
The doc contains a meaningless placeholder line.

### Modifications:
Removed the placeholder line.

### Result:
Syntax errors are removed from demo code snippet.

Author: EmingK

Sources/PackageManagerDocs/Documentation.docc/Plugins/WritingCommandPlugin.md

@@ -83,7 +83,6 @@ import Foundation
 
 @main
 struct MyCommandPlugin: CommandPlugin {
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   func performCommand(
     context: PluginContext,
     arguments: [String]