utils/curl: fix Digest::SHA256 typo

Should fix the error seen in Homebrew/homebrew-core#153647 [^1]. This
was introduced in e5d656b and `Digest::256` seems to be the intended
one.

[^1]: https://github.com/Homebrew/homebrew-core/actions/runs/6805354565/job/18504732312?pr=153647#step:4:36

Library/Homebrew/utils/curl.rb

@@ -456,7 +456,7 @@ def curl_http_content_headers_and_checksum(
           end
         end
         file_contents = File.read(T.must(file.path), **open_args)
-        file_hash = Digest::SHA2.hexdigest(file_contents) if hash_needed
+        file_hash = Digest::SHA256.hexdigest(file_contents) if hash_needed
       end
 
       {