Use the SKS port 80 keyserver pool, as it is more reliable than the MIT server

Signed-off-by: Justin Cormack <justin@specialbusservice.com>

docs/installation/debian.md

@@ -51,7 +51,7 @@ from the new repository:
 
  4. Add the new `gpg` key.
 
-         $ apt-key adv --keyserver hkp://pgp.mit.edu:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
+         $ apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
 
  5. Open the `/etc/apt/sources.list.d/docker.list` file in your favorite editor.
 

docs/installation/ubuntulinux.md

@@ -55,7 +55,7 @@ packages from the new repository:
 
 3. Add the new `gpg` key.
 
-        $ sudo apt-key adv --keyserver hkp://pgp.mit.edu:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
+        $ sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
 
 4. Open the `/etc/apt/sources.list.d/docker.list` file in your favorite editor.