fix: fix typo

Author: leverimmy

chap4/part1.tex

@@ -304,7 +304,7 @@ \subsubsection{系统的频率响应}
             \path [line] (circy) -- (zy1);
 
             \path [line] (input) -- (sum);
-            \node [gainy, below of=zy1, xshift=-2cm, yshift=-0.5cm] (zgy1) {$1$};
+            \node [gainy, below of=zy1, xshift=-2cm, yshift=-0.5cm] (zgy1) {$a$};
             \path [line] (zy1) |- (zgy1);
             \path [line] (zgy1.west) -- (sum);
         \end{tikzpicture}