doc: 修正快速使用手册中的\isGraduateTF

1. 之前的环境变量根本影响`\isGraduateTF`结果，始终用`#2`。这次将`if_eq`从 token list 的换成 string 的，并将`\GRADUATE`从`n`（token list）改为`V`（value of the command）。

    > …we can choose between `\tl_if_eq:` (from token list library) and `\str_if_eq:` (from string library). Unless **it is absolutely necessary, it is recommended to use string library’s comparision functions**. That i1s because `\tl_if_eq:` not only checks if the characters are the same, but it also checks if the category code of each character is the same. As a result, two seemingly identical variables can result in `False` outcome when using `\tl_if_eq:`.
    >
    > —[LaTeX3: Programming in LaTeX with Ease | Alan Xiang’s Blog](https://www.alanshawn.com/tech/2020/10/04/latex3-tutorial.html)

2. 命令内部不能使用`lstlisting`环境。这里用`\lstinputlisting`绕开了。

    这种情况只在`\isGraduateTF`的`#1`里有，之前测不到。

Author: YDX-2147483647

the-graduates-handbook/chapters/ch2-example-pub.bib

@@ -0,0 +1,9 @@
+@article{zhang2021,
+  author    = {张三 and 李四 and 王五},
+  title     = {论文标题},
+  journal   = {期刊名},
+  year      = {2021},
+  volume    = {1},
+  pages     = {1--10},
+  author+an = {1:myself="\Author"},
+}

the-graduates-handbook/chapters/ch2-template-usage.tex

@@ -177,17 +177,7 @@ \section{其他部分}
 根据 pub.bib 中的示例以及注释说明，为每个文献条目添加并设置
  \texttt{author+an} 字段。
 比如，如果作为张三的你的文献条目为：
-\begin{lstlisting}[language=TeX]
-@article{zhang2021,
-  author = {张三 and 李四 and 王五},
-  title = {论文标题},
-  journal = {期刊名},
-  year = {2021},
-  volume = {1},
-  pages = {1--10},
-  author+an = {1:myself="\Author"},
-}
-\end{lstlisting}
+\lstinputlisting[language=TeX]{chapters/ch2-example-pub.bib}
 通过设置 \texttt{author+an} 字段，我们可以在盲审模式打开时，
 将自己的名字自动替换为「第一作者」。
 %

the-graduates-handbook/main.tex

@@ -92,8 +92,12 @@
  \tl_gset:Nn \GRADUATE {}
  \getenv[\GRADUATE]{GRADUATE}
 
+  % 若为研究生，显示第一个参数；若为本科生，显示第二个参数。
+  % 命令内部不能使用 lstlisting 环境，原因如下。可用 \lstinputlisting 绕开。
+  % Verbatim material can't be placed in a macro argument because then the material got already parsed by TeX and the switching to verbatim mode comes to late.
+  % https://tex.stackexchange.com/a/122055
  \NewDocumentCommand{\isGraduateTF}{ m m } {
-   \tl_if_eq:nnTF {\GRADUATE} { true } { #1 } { #2 }
+   \str_if_eq:VnTF \GRADUATE { true } { #1 } { #2 }
  }
 
 \ExplSyntaxOff