CI: extend spellchecker whitelist, add "ists" as well

codespell does not handle plurals, we already whitelusted "ist", let us
whitelist "ists" as well
diff --git a/.github/workflows/codespell.yml b/.github/workflows/codespell.yml
index c2130eb..de49f43 100644
--- a/.github/workflows/codespell.yml
+++ b/.github/workflows/codespell.yml
@@ -14,4 +14,4 @@
     - name: install prerequisites
       run: sudo pip install codespell
     - name: check
-      run: codespell -c -q 2 --ignore-words-list ist,hist,wan,ca,cas,que,ans,te,nd,referer,ot,uint,iif,fo,keep-alives,dosen --skip="CHANGELOG,Makefile,*.fig,*.pem"
+      run: codespell -c -q 2 --ignore-words-list ist,ists,hist,wan,ca,cas,que,ans,te,nd,referer,ot,uint,iif,fo,keep-alives,dosen --skip="CHANGELOG,Makefile,*.fig,*.pem"