Correct link to 'Eligible snippets' headline

[sten0]

I've confirmed that this patch produces a functioning link using Emacs 27.1 which bundles org-mode 9.3. Without it, generating HTML docs with emacs -Q -L /usr/share/emacs/site-lisp/elpa-src/htmlize-* -L . --batch -l htmlize -l doc/yas-doc-helper.el -f yas--generate-html-batch fails with

Code block evaluation complete.
Publishing file /<<PKGBUILDDIR>>/doc/snippet-expansion.org using ‘org-html-publish-to-html’
Unable to resolve link: "Eligible%20snippets"

Were I to guess, I'd guess that encoding the space was necessary in past org-mode releases. This appears to no longer be necessary because a valid <a href="#org5b8a14e"> link is generated now.