Move installmirrors anchor in doc

so it's before "Mirrors" instead of "Docker and local DNS server
warnings"

Docker-DCO-1.1-Signed-off-by: Marc Abramowitz <msabramo@gmail.com> (github: msabramo)

docs/sources/installation/ubuntulinux.rst

@@ -282,8 +282,6 @@ incoming connections on the Docker port (default 4243):
 
    sudo ufw allow 4243/tcp
 
-.. _installmirrors:
-
 Docker and local DNS server warnings
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -342,6 +340,8 @@ NetworkManager and Docker need to be restarted afterwards:
 
 .. warning:: This might make DNS resolution slower on some networks.
 
+.. _installmirrors:
+
 Mirrors
 ^^^^^^^