Fix broken link in control_flow.md (#1942)

This currently sends you to "https://www.nushell.sh/book/book/thinking_in_nu.html" instead of "https://www.nushell.sh/book/thinking_in_nu.html".

Author: axelcool1234

book/control_flow.md

@@ -156,7 +156,7 @@ $result
 # => ╰───┴───╯
 ```
 
-While it may be tempting to use loops if you're familiar with them in other languages, it is considered more in the [Nushell-style](book/thinking_in_nu.html) (idiomatic) to use commands that apply closures when you can solve a problem either way. The reason for this is because of a pretty big downside with using loops.
+While it may be tempting to use loops if you're familiar with them in other languages, it is considered more in the [Nushell-style](/book/thinking_in_nu.html) (idiomatic) to use commands that apply closures when you can solve a problem either way. The reason for this is because of a pretty big downside with using loops.
 
 #### Loop Disadvantages