HistoryBlobCurStub: correct setting name in doc comment

It is $wgLegacySchemaConversion = true that causes these to be
inserted into the text table when upgrading from 1.4 or earlier,
and this is the only occurrence of $wgFastSchemaUpgrades even on
the REL1_5 branch.

Change-Id: I57634343d3515a4fb982c648e22ad23c2af0f2f6
Merge "HistoryBlobCurStub: correct setting name in doc comment"