Update nimscript withDir doc example (#19776)

lib/system/nimscript.nim

@@ -342,17 +342,17 @@ template withDir*(dir: string; body: untyped): untyped =
   ## Usage example:
   ##
   ## .. code-block:: nim
+  ##   # inside /some/path/
   ##   withDir "foo":
-  ##     # inside foo
-  ##   #back to last dir
-  var curDir = getCurrentDir()
+  ##     # move to /some/path/foo/
+  ##   # back in /some/path/
+  let curDir = getCurrentDir()
   try:
     cd(dir)
     body
   finally:
     cd(curDir)
 
-
 proc writeTask(name, desc: string) =
   if desc.len > 0:
     var spaces = " "