Fix deprecated dotnet new install syntax in documentation

[Copilot]

Updates the README.md documentation to use the current @ syntax instead of the deprecated :: syntax for specifying package versions in dotnet new install commands.

Background

The .NET CLI has deprecated the colon separator :: for package version specification in favor of the @ symbol. This change was introduced to align with standard package management conventions and improve consistency across .NET tooling.

Changes

The following command examples have been updated:

Before:

dotnet new install NetCorePal.Template::<package-version> --add-source "https://www.myget.org/F/netcorepal/api/v3/index.json"
dotnet new install NetCorePal.Template::<VERSION>

After:

dotnet new install NetCorePal.Template@<package-version> --add-source "https://www.myget.org/F/netcorepal/api/v3/index.json"
dotnet new install NetCorePal.Template@<VERSION>

These changes ensure users won't encounter deprecation warnings when following the documentation and align with the current .NET CLI best practices.

Fixes the issue where users would see deprecation warnings when using the installation commands from the documentation.

Original prompt

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This section details on the original issue you should resolve

<issue_title>fix docs</issue_title>
<issue_description>已弃用冒号分隔符 "::"，以支持在 dotnet 新安装中使用符号 “@” 分隔程序包和版本。在你的案例中，这意味着使用 NetCorePal.Template@3.0.0-preview.1 而不是 NetCorePal.Template::3.0.0-preview.1。</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #84

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