Fix weak coupling approximation in LSH tutorial

[Michael Lip (theluckystrike)]

In the Approximation 1 background section the tutorial states that in the weak-coupling regime the dynamics is dominated by the electric term H_E, which favors states with large n_l. That contradicts the Hamiltonian given earlier in the same notebook. With W = H_E + mu H_M + x H_I a large x makes the off-diagonal interaction term x H_I dominant, matching the source paper. It is the strong-coupling regime where H_E dominates and the vacuum has n_l = 0.

The large n_l in this tutorial comes from a separate modeling choice, a large incoming boundary flux that makes the n_l dependent hopping prefactors approach one.

This rewrites that one sentence in the Approximation 1 paragraph. No code cells are touched.

Closes #5666

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