add help cd;fix bug:cd into a relative path

static/help/cd.txt

@@ -0,0 +1,5 @@
+cd - changes the working directory<br/>
+if the directory doesn't exist or the directory is a file, it will throw an error<br/>
+if no directory is specified, it will change the working directory to the previous directory<br/>
+if the directory is a relative path, it will change the working directory to the relative path from the current directory<br/>
+if the directory is an absolute path, it will change the working directory to the absolute path<br/>

static/terminal.php/help.txt

@@ -1,5 +1,6 @@
 clear - clears the screen<br/>
 pwd - shows the working directory<br/>
 mkdir - creates a new directory<br/>
+cd - changes the working directory<br/>
 help - displays this help message<br/>
 type help [command] to show more details<br/>

terminal.php

@@ -8,7 +8,7 @@ if (file_get_contents("pwd.txt") == NULL) {
     file_put_contents("pwd.txt", "/");
 }
 $pwd = file_get_contents("pwd.txt");
-if(!opendir("fileroot" . $pwd)){
+if (!opendir("fileroot" . $pwd)) {
     !file_put_contents("pwd.txt", "/");
 }
 if (!opendir("fileroot")) {
@@ -52,10 +52,12 @@ if (!opendir("fileroot")) {
         }
     } else if ($commands[0] == "clear" & sizeof($commands) == 1) {
         file_put_contents("recent.txt", "");
-    } else if ($commands[0] == "cd" & sizeof($commands) == 2) {
-        if ($commands[1] == "..") {
+    } else if ($commands[0] == "cd") {
+        if ($commands[1] == ".." || sizeof($commands) == 1) {
             $pwd = substr($pwd, 0, strrpos($pwd, "/"));
             file_put_contents("pwd.txt", $pwd);
+        } else if (str_starts_with($commands[1], "/")) {
+            file_put_contents("pwd.txt", $commands[1]);
         } else {
             if (opendir("fileroot" . $pwd . "/" . $commands[1])) {
                 if ($pwd == "/")