process: Author ↦ Authors

```
sed -z -i 's/Author: /Authors: /' bip-0*.md
sed -z -i 's/Author: /Authors: /' bip-0*.mediawiki
```

Also align correctly in case of multiple authors.

bip-0375.mediawiki

@@ -2,9 +2,9 @@
   BIP: 375
   Layer: Applications
   Title: Sending Silent Payments with PSBTs
-  Author: Andrew Toth <andrewstoth@gmail.com>
-          Ava Chow <me@achow101.com>
-          josibake <josibake@protonmail.com>
+  Authors: Andrew Toth <andrewstoth@gmail.com>
+           Ava Chow <me@achow101.com>
+           josibake <josibake@protonmail.com>
   Status: Draft
   Type: Specification
   Created: 2025-01-08