bert: Fix bigbird_public mount path

The mount path had changed months back:

    commit ccc8708a6b
    Author: Chandler Swift <chandler@chandlerswift.com>
    Date:   Tue Jan 14 23:31:14 2025 -0600

        bert: Rename mount so systemd doesn't escape hyphen

    diff --git a/bert/configuration.nix b/bert/configuration.nix
    index 61f79b1..4072b87 100644
    --- a/bert/configuration.nix
    +++ b/bert/configuration.nix
    @@ -40 +40 @@
    -  fileSystems."/mnt/bigbird-public" = {
    +  fileSystems."/mnt/bigbird_public" = {

but I missed this use, so `/sheets` was 404ing.

bert/services/http/home.chandlerswift.com.nix

@@ -14,7 +14,7 @@
       # vulnerable to a directory traversal attack (and it doesn't really
       # matter anyway; everything in there is public somewhere or another!) but
       # it sorta feels wrong to do this without a `/sheets` suffix.
-      root /mnt/bigbird-public
+      root /mnt/bigbird_public
     }
 
     handle_errors {