mkdocs-offline.yml: fix a typo, forgot a word

mkdocs-offline.yml

@@ -1,6 +1,6 @@
 # This config is good if you plan to use the rendered docs from
 # your file system. To use, run:
-#      poetry run -- mkdocs -f mkdocs-offline.yml
+#      poetry run -- mkdocs build -f mkdocs-offline.yml
 INHERIT: 'mkdocs.yml'
 plugins:
     - offline