Update arcane surge cast speed calc from 10% to 20% for 3.26 patch

[Ariacell]

Description of the problem being solved:

One liner to update arcane surge cast speed modifier calculation ahead of 3.26 based on patch notes. Feel free to close if there's a convention of waiting until closer to the release to start making these changes! Just getting my feet wet confirming my dev environment was set up correctly 😄

Evidence for change: https://www.pathofexile.com/forum/view-thread/3787013

Steps taken to verify a working solution:

Validated minimal case by attaching arcane surge to a sample build and confirming cast rate is impacted appropriately.

Tests pass (although as far as I can tell no direct coverage for this mod? I can look into adding some if preferred)

Before screenshot:

After screenshot: