Add change 33 to instructions

how_to/Change_33.md

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+data: Dereference refs when reading and writing
+
+Now we'll dereference symbolic refs not only when reading them but also when
+writing them.
+
+We'll implement a helper function called `_get_ref_internal` which will return
+the path and the value of the last ref pointed by a symbolic ref. In simple words:
+
+- When given a non-symbolic ref, `_get_ref_internal` will return the ref name
+and value.
+- When given a symbolic ref, `_get_ref_internal` will dereference the ref
+recursively, and then return the name of the last (non-symbolic) ref that points
+to an OID, plus its value.
+
+Now `update_ref` will use `_get_ref_internal` to know which ref it needs to update.
+
+Additionally, we'll use `_get_ref_internal` in `get_ref`.