[fix](doc) dev: fix array_count nested higher-order example (EN)

[boluor]

What

The dev EN array-count page presented this nested higher-order example as if it works, claiming a result of 2:

SELECT array_count(x -> array_exists(y -> y > 5, x), [[1,2,3],[4,5,6],[7,8,9]]);

But array_exists returns an ARRAY (one boolean per inner element), and array_count expects the lambda body to return a scalar it can cast to BOOLEAN. On a live cluster this errors:

ERROR 1105 (HY000): errCode = 2, detailMessage =
  Can not find the compatibility function signature: array_count(ARRAY<ARRAY<BOOLEAN>>)

Fix

This is intended behavior, not a signature gap — the dev ZH, v4.x ZH and v4.x EN pages already document this exact error as the "lambda returns an array" failure case. This PR brings dev EN in line with the v4.x EN treatment: show the signature error, then give a working alternative whose lambda returns a scalar BOOLEAN:

SELECT array_count(x -> size(array_filter(y -> y > 5, x)) > 0, [[1,2,3],[4,5,6],[7,8,9]]);
-- 2

Verified live

Both statements run on a master daily build (doris-0.0.0-2e72603618c): the first reproduces the signature error verbatim, the second returns 2.

Scope

EN-only. dev ZH, v4.x EN and v4.x ZH already document the error correctly, so no backport is needed.

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