Bump vtk from 9.2.5 to 9.2.6 in /requirements (#215)

Bumps [vtk](https://vtk.org) from 9.2.5 to 9.2.6.

---
updated-dependencies:
- dependency-name: vtk
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements/requirements_docs.txt

@@ -6,4 +6,4 @@ pyvista==0.38.3
 sphinx-copybutton==0.5.1
 sphinx-gallery==0.11.1
 sphinx-notfound-page==0.8.3
-vtk==9.2.5
+vtk==9.2.6