Fix bibtex entry type for the mobility working paper

The `@techreport` entry for carroll2026mobility had no `institution` field,
which pybtex flags as "missing institution" — and the CI build runs with `-W`,
so the single warning failed the build.

The paper has no issuing series or working paper number, so naming an
institution would be inventing one. `@unpublished` with a `note` is the
accurate entry type and matches how Greenfield_Sargent_1993 is recorded.

Verified with pybtex across the alpha, unsrt and plain styles: no warnings for
this entry or the other 14 added in the previous commit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: jstac

lectures/_static/quant-econ.bib

@@ -2978,13 +2978,11 @@ @article{decock2011ames
   year={2011}
 }
 
-@techreport{carroll2026mobility,
-  title       = {Mobility},
-  author      = {Carroll, Daniel R. and Hoffman, Nicholas and Young, Eric R.},
-  year        = {2026},
-  type        = {Working Paper},
-  note        = {Federal Reserve Bank of Cleveland, Carnegie Mellon University
-                 and University of Virginia}
+@unpublished{carroll2026mobility,
+  title  = {Mobility},
+  author = {Carroll, Daniel R. and Hoffman, Nicholas and Young, Eric R.},
+  year   = {2026},
+  note   = {Working paper, Federal Reserve Bank of Cleveland, Carnegie Mellon University and University of Virginia}
 }
 
 @article{shorrocks1978measurement,