Update nbins in PoldiAutoCorrelation doc test

[RichardWaiteSTFC]

Description of work

Doc test was failing

document: algorithms/PoldiAutoCorrelation-v6
--------------------------------------------
# stuff...
Expected:
    The workspace contains 448 spectra.
    The correlation spectrum has 2460 bins.
Got:
    The workspace contains 448 spectra.
    The correlation spectrum has 2833 bins.
**********************************************************************
1 items had failures:
   1 of   1 in POLDI_silicon_autocorr
1 tests in 1 items.
0 passed and 1 failed.
***Test Failed*** 1 failures.

This is after a bug was fixed in max arrival time calculation in PR #39912 in commit 704e68a
The value now matches the unit test value.

To test:

(1) Run doc tests locally

No release note required as not user facing change

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