process: Post-History ↦ Discussion

Also line up with additional items in the lines below.

```
sed -i -z 's/  Post-History: /  Discussion:   /' bip-0*.md
sed -i -z 's/  Post-History: /  Discussion:   /' bip-0*.mediawiki
```

bip-0099.mediawiki

@@ -8,7 +8,7 @@
   Type: Informational
   Created: 2015-06-20
   License: PD
-  Post-History: http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-June/008936.html
+  Discussion: http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-June/008936.html
 </pre>
 
 ==Abstract==