docs(migration[currentmodule]): Fix incorrect module name (libtmux -> libvcs)

why: Stale copy-paste from libtmux's migration doc; sphinx currentmodule
should be libvcs.
what:
- docs/migration.md: `{currentmodule} libtmux` -> `{currentmodule} libvcs`

Salvaged from #484 (closed); rest of that PR was superseded by master's
gp-sphinx migration and uv adoption.

Author: tony

docs/migration.md

@@ -1,6 +1,6 @@
 (migration)=
 
-```{currentmodule} libtmux
+```{currentmodule} libvcs
 
 ```