doc/build-helpers: add missing semicolons (#298632)

Co-authored-by: seksea <[e-mail]>

doc/build-helpers/trivial-build-helpers.chapter.md

···
     echo "hi"
   '';
   executable = true;
-  destination = "bin/my-script"
+  destination = "bin/my-script";
 }
 ```
 
···
     echo "hi"
   '';
   executable = true;
-  destination = "bin/my-script"
+  destination = "bin/my-script";
 }
 ```