Mistake in documentation of step-44

[LucasCampos]

I believe there is a minor mistake on the documentation of step-44 of the Tutorial. Namely, in the note that starts with "The simplified single-field derivation ( u is the only primary variable) below makes it clear how we transform the limits of integration to the reference domain", the first integral should read

\int_{\Omega}\delta \mathbf{u} \cdot [\textrm{div}\ \boldsymbol{\sigma} + \mathbf{b}^\text{p}]~\mathrm{d}v

That is, there is a missing div before the sigma, which comes directly from the equilibrium equation.