manual: Replace FTP link to libgcrypt.

Apparently, this FTP server has been switched off and checklink is
right to complain about it. Use the https link to the web page instead.

diff --git a/web/manual.md b/web/manual.md
index 0db1e22c26fdee83b3df4aaf60bad0537e40a0f1..d12c4917a61d2bad2c188253beb91de5652a08fc 100644 (file)
--- a/web/manual.md
+++ b/web/manual.md
@@ -331,7 +331,7 @@ code repository, execute
                git clone https://git.tuebingen.mpg.de/osl
 
 - [openssl](https://www.openssl.org/) or
-[libgcrypt](ftp://ftp.gnupg.org/gcrypt/libgcrypt/).  At least one
+[libgcrypt](https://directory.fsf.org/project/libgcrypt/). At least one
 of these two libraries is needed as the backend for cryptographic
 routines on both the server and the client side. Both openssl and
 libgcrypt are usually shipped with the distro, but you might have