fix(typo): update typo with code in script for initiating proxy configuration for terminal or shell

docs/development/proxy4shell-terminal.md

@@ -123,7 +123,7 @@ cat >> ~/.zshrc << EOF
 # session proxy switch shortcut
 function proxy_on() {
     export http_proxy=http://127.0.0.1:7890
-    export https_proxy=\$http_proxy
+    export https_proxy=$http_proxy
     echo -e "global proxy for the current terminal session has turned on"
 }
 
@@ -233,4 +233,4 @@ ssh -o ProxyCommand='socat - "PROXY:%h:%p|tcp:myproxy:80"' user@host
 -   https://askubuntu.com/questions/583797/how-to-set-a-proxy-for-terminal
 -   https://zhuanlan.zhihu.com/p/357875811
 -   https://askubuntu.com/questions/158557/setting-proxy-for-apt-from-terminal
-    :::
+    :::