[Backward] Fix trapezoidal rule in EulerImplicitSolver

[hugtalbot]

Fix #5140

NB: it would be nice to have the image of the issue added (as text) in the doc

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[alxbilger]

Here is what I have for Backward Euler:

$$ \left( \left( 1 + \alpha \Delta t \right) M - \Delta t B - \Delta t \left( \Delta t + \beta \right) K \right) \Delta v = \Delta t F(x_n, v_n) + \Delta t \left( -\alpha M + \left( \beta + \Delta t \right) K \right) v_n $$

and for the trapezoidal rule:

$$ \left[ \left(1 + \frac{1}{2} \alpha \Delta t \right) M - \frac{1}{2} \Delta t B - \frac{1}{2} \Delta t \left( \frac{1}{2} \Delta t + \beta \right) K \right] \Delta v = \Delta t F(x_n, v_n) + \Delta t \left( -\alpha M + \left( \beta + \frac{1}{2} \Delta t \right) K \right) v_n $$

So, your changes make sense to me