Add goto-transcoder.md into summary (model-checking#243)

The goto-transcoder is not appearing in the book pages. This should fix
it.

Author: rafaelsamenezes

doc/src/SUMMARY.md

@@ -8,6 +8,7 @@
 
 - [Verification Tools](./tools.md)
   - [Kani](./tools/kani.md)
+  - [GOTO Transcoder](./tools/goto-transcoder.md)
 
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