chore: bump the pinned theme to v2.6.1

Patch release carrying the fix for QuantEcon/quantecon-theme.mystmd#186:
on the Netlify preview, pressing Back after following an in-page anchor
replaced the page with an "Application Error" screen, and the "Top"
control did a full reload. No configuration changes are needed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: mmcky

lectures/myst.yml

@@ -93,4 +93,4 @@ site:
   # Pinned theme release (see https://github.com/QuantEcon/quantecon-theme.mystmd/releases);
   # bump the vX.Y.Z in this URL to take a new theme version. The old bundled
   # QuantEcon/quantecon-theme repo is archived.
-  template: https://github.com/QuantEcon/quantecon-theme.mystmd/releases/download/v2.6.0/quantecon-theme.zip
+  template: https://github.com/QuantEcon/quantecon-theme.mystmd/releases/download/v2.6.1/quantecon-theme.zip