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
```

scripts/buildtable.pl

@@ -20,7 +20,7 @@ my %MayHaveMulti = (
 	'Comments-URI' => undef,
 	License => undef,
 	'License-Code' => undef,
-	'Post-History' => undef,
+	'Discussion' => undef,
 );
 my %DateField = (
 	Created => undef,
@@ -32,7 +32,7 @@ my %EmailField = (
 my %MiscField = (
 	'Comments-Summary' => undef,
 	'Discussions-To' => undef,
-	'Post-History' => undef,
+	'Discussion' => undef,
 	'Replaces' => undef,
 	'Requires' => undef,
 	'Proposed-Replacement' => undef,