ci(update-book): the English version owns /book/index.html

The default language of the ProGit book is English, therefore
https://git-scm.com/book should redirect to the table of contents of the
English version of that book.

This means that the `/external/book/content/book/en/_index.html` file
needs to be part of the sparse checkout, otherwise the workflow run
would not be able to update it (should it ever become necessary).

This was not a problem so far because that file remained unchanged (and
is likely to remain so for quite some time yet).

Signed-off-by: Johannes Schindelin <[email protected]>

.github/workflows/update-book.yml

@@ -55,6 +55,7 @@ jobs:
             external/book/sync
             external/book/data
             external/book/content/book/${{ matrix.language.lang }}
+            external/book/content/book${{ matrix.language.lang != 'en' && '/en' || '' }}
             external/book/static/book/${{ matrix.language.lang }}
       - name: clone ${{ matrix.language.repository }}
         run: |