Update nginx.docs-backend.conf (#312)

The `^~` modifier makes this rule take priority priority over 
regex (specifically the `pdf|zip|epub|bz2` regex above),
see https://nginx.org/en/docs/http/ngx_http_core_module.html#location

salt/docs/config/nginx.docs-backend.conf

@@ -32,7 +32,7 @@ server {
     }
 
     # Some doc download pages link to docs.python.org/ftp instead of www.python.org/ftp.
-    location /ftp/ {
+    location ^~ /ftp/ {
         return 301 https://www.python.org$request_uri;
     }