The Stacks project

Proof. Observe that $\mathcal{O}_{U, u_0}$ is a Noetherian local $S$-algebra with residue field $k$. Hence $R = \mathcal{O}_{U, u_0}^\wedge $ is an object of $\mathcal{C}_\Lambda ^\wedge $, see Formal Deformation Theory, Definition 90.4.1. Recall that $\xi $ is versal if $\underline{\xi } : \underline{R}|_{\mathcal{C}_\Lambda } \to \mathcal{F}_{\mathcal{X}, k, x_0}$ is smooth and $x$ is versal at $u_0$ if $\hat x : \mathcal{F}_{(\mathit{Sch}/U)_{fppf}, k, u_0} \to \mathcal{F}_{\mathcal{X}, k, x_0}$ is smooth. There is an identification of predeformation categories

see Formal Deformation Theory, Remark 90.7.12 for notation. Namely, given an Artinian local $S$-algebra $A$ with residue field identified with $k$ we have

Unwinding the definitions the reader verifies that the resulting map

is equal to $\underline{\xi }$ and we see that the lemma is true. $\square$