fixed typo 'isn't is?' -> 'isn't it?' in test_book/src/individual/code.md

Author: Sableoxide

test_book/src/individual/code.md

@@ -17,7 +17,7 @@ This line contains `inline code` mixed with some other stuff. (LTR)
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 ````
-escaping ``` in ```, fun, isn't is?
+escaping ``` in ```, fun, isn't it?
 ````
 
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