Fix PolarizerEfficiency Equation

[cailafinn]

This is a fix for a bug in code being introduced to this release. It should not be swapped to the 6.12 milestone after code freeze! Will need its base changing to release-next once it's been created.

Description of work

Summary of work

The equation had some incorrect terms in the calculation that was making a mess of the output. It was approximately half of what it should have been. This corrects that to produce the right outputs.

Fixes #37939

Further detail of work

To test:

1. The honest best test for this is to use Dirk's script and check the outputs as it shows how the algorithm will be used in practice. The problem is that it's very long and requires some specific zoom files. When reviewing, just ask me for them. I'll send you a .zip.
2. Unpack the zip and add the directory into your User directories.
3. Open and Run the script. It'll fail early, but you'll have the workspaces you need.
4. Plot MantidAlgorithm_efficiencyPolarizer, efficiency_polarizer_mantidlike and efficiency_polarizer_pa
5. The MantidAlgorithm and mantidlike plots should match.

This does not require release notes because it fixes an issue in an algorithm added for this release.

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[rbauststfc]

The calculation change looks good and appropriate docs have been updated. I've tested the result using Dirk's script and it's giving the expected result, so this is all looking good 👍