adding indentation

Author: HumphreyYang

lectures/python_essentials.md

@@ -332,9 +332,9 @@ We can also use a `with` statement (also known as a [context manager](https://re
 ```{code-cell} python3
 with open("newfile.txt", "r") as f:
     file = f.readlines()
-with open("output.txt", "w") as fo:
-    for i, line in enumerate(file):
-        fo.write(f'Line {i}: {line} \n')
+    with open("output.txt", "w") as fo:
+        for i, line in enumerate(file):
+            fo.write(f'Line {i}: {line} \n')
 ```
 
 ```{code-cell} python3