gnu: asciidoc: Update source URI.

* gnu/packages/documentation.scm (asciidoc)[source]: Update URI.

This fixes <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=46706>.

They moved (and redirected) the repository to the new location
<https://github.com/asciidoc-py/asciidoc-py>. Here they changed the old
commits in-place using now Python3, which caused the hash mismatch.

The untouched Python2 version is still found under the ..-py2 repository.

gnu/packages/documentation.scm

@@ -105,7 +105,7 @@ pages in HTML.")
     (source (origin
               (method git-fetch)
               (uri (git-reference
-                     (url "https://github.com/asciidoc/asciidoc")
+                     (url "https://github.com/asciidoc-py/asciidoc-py2")
                      (commit version)))
               (file-name (git-file-name name version))
               (sha256