fix: correct stream redirection exercise

The previous solution was confusing. The argument 2 could be interpreted as a redirection argument instead of the argument to the command cal.

7_streams_and_pipes.Rmd

@@ -101,7 +101,7 @@ What happened ? Can you check the content of `my_notes` ? How would you modify t
 Finally, you can redirect a stream toward another stream with the following syntax:
 
 ```sh
-cal -N2 2&> my_redirection
+cal -N 2 &> my_redirection
 cal 2 &>> my_redirection
 ```