Fix link in basics TOC to sumstruct

This link seems broken.

Closes #44
https://github.com/cuelang/cue/pull/44

GitOrigin-RevId: 6ce2bb7f797fda2c5f7cf635fac9e50611a1b759
Change-Id: I38c82ec392e4cbca53000be69fc1192702452fe9
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2080
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/doc/tutorial/basics/Readme.md b/doc/tutorial/basics/Readme.md
index 3628a43..e133552 100644
--- a/doc/tutorial/basics/Readme.md
+++ b/doc/tutorial/basics/Readme.md
@@ -37,7 +37,7 @@
   - [Order is Irrelevant](unification.md)
   - [Disjunctions](disjunctions.md)
   - [Default Values](defaults.md)
-  - [Disjunctions of Structs](disjstruct.md)
+  - [Disjunctions of Structs](sumstruct.md)
   - [Numbers](numbers.md)
   - [Ranges](ranges.md)
   - [Predefined Ranges](rangedef.md)