fix mesa link

The ftp address is out-dated.

Author: tomguluson92

docs/source/install/index.rst

@@ -81,7 +81,7 @@ First, install build dependencies via `apt` or your system's package manager.
 Then, download the current release of Mesa from here_.
 Unpack the source and go to the source folder:
 
-.. _here: ftp://ftp.freedesktop.org/pub/mesa/mesa-18.3.3.tar.gz
+.. _here: https://archive.mesa3d.org/mesa-18.3.3.tar.gz
 
 .. code-block:: bash