gtk3: Use GNOME domain name

Use the domain name used in the book *Linux from Scratch* [1].

It’s the same system.

```
$ host ftp.acc.umu.se
ftp.acc.umu.se has address 194.71.11.165
ftp.acc.umu.se has address 194.71.11.173
ftp.acc.umu.se has IPv6 address 2001:6b0:19::173
ftp.acc.umu.se has IPv6 address 2001:6b0:19::165
ftp.acc.umu.se mail is handled by 0 mail.acc.umu.se.
$ host ftp.gnome.org
ftp.gnome.org is an alias for ftp.acc.umu.se.
ftp.acc.umu.se has address 194.71.11.165
ftp.acc.umu.se has address 194.71.11.173
ftp.acc.umu.se has IPv6 address 2001:6b0:19::173
ftp.acc.umu.se has IPv6 address 2001:6b0:19::165
ftp.acc.umu.se mail is handled by 0 mail.acc.umu.se.
```

[1] http://www.linuxfromscratch.org/blfs/view/svn/x/gtk3.html

gtk3.be0

@@ -9,7 +9,7 @@
 ## The source URL(s) define the location of the sources that will be
 ## downloaded. Version variables may be used to simplify reuse of this bee-file.
 
-SRCURL[0]="https://ftp.acc.umu.se/pub/gnome/sources/gtk+/${PKGVERSION[2]}/gtk+-${PKGVERSION}.tar.xz"
+SRCURL[0]="https://ftp.gnome.org/pub/gnome/sources/gtk+/${PKGVERSION[2]}/gtk+-${PKGVERSION}.tar.xz"
 
 ###############################################################################
 ## Add URLs/pathes to patch files to the PATCHURL array.