Fix Nullable type handling in Lua docs.

[pchote]

Fixes #19178.

Before:

<tr><td align="right" width="50%"><strong>void DisplayMessage(string text, string prefix = Mission, Nullable`1 color = nil)</strong></td><td>Display a text message to the player.</td></tr>

After:

<tr><td align="right" width="50%"><strong>void DisplayMessage(string text, string prefix = Mission, Color? color = nil)</strong></td><td>Display a text message to the player.</td></tr>

[pchote]

Ping for a second review.

[teinarss]

https://github.com/OpenRA/OpenRA/wiki/Changelog-(bleed)/_compare/6966f02c3034c47afb341182f2ad1bc8913f081f...0d8e6e1d8332265a331814f6e9c6fd49c86bc911