doc: Move "This is equivalent to" into example block

This patch moves the code snippet which is shown as equivalent to the
example into the actual example block.

Visually, it was not easy to parse that the "equivalent" code piece
belonged to the code snippet that was hidden in the example (which is
collapsed by default).

By moving it into the example block, the "equivalent" piece is hidden by
default as well.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

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

@@ -533,7 +533,6 @@ writeScript "my-file"
   Contents of File
   ''
 ```
-:::
 
 This is equivalent to:
 
@@ -546,6 +545,7 @@ writeTextFile {
   executable = true;
 }
 ```
+:::
 
 ### `writeScriptBin` {#trivial-builder-writeScriptBin}