report: update citation

We use the Extended Report which actually contains example
applications which is exactly what I will do next.

doc/report/references.bib

@@ -81,10 +81,10 @@
 }
 
 @online{tetra,
-    title={Towards a Heterogeneous Query Language for Mathematical Knowledge},
+    title={Towards a Heterogeneous Query Language for Mathematical Knowledge: Extended Report},
     author={Bercic, Katja and Kohlhase, Michael and Rabe, Florian},
-    urldate = {2020-06-16},
-    url = {https://kwarc.info/people/mkohlhase/papers/cicm20-search.pdf},
+    urldate = {2020-06-24},
+    url = {https://kwarc.info/people/mkohlhase/papers/tetrasearch.pdf},
 }
 
 @book{owl,