fix: correct field extractor exercise

Improved the solution of the exercise by filtering line with type transcript then focusing on the last column.

8_text_manipulation.Rmd

@@ -239,7 +239,7 @@ You can change the field separator with the option `-d`, set it to `";"` to extr
 <details><summary>Solution</summary>
 <p>
 ```sh
-gzip -dc hg38.ncbiRefSeq.gtf.gz | head | cut -f 2 -f 5 -d ";"
+gzip -dc hg38.ncbiRefSeq.gtf.gz | head | grep -w "transcript" | cut -f 9 | cut -f 2,3 -d ";"
 ```
 </p>
 </details>