Error on FocalPointPlasticity's custom LinkConstituentLaw example

[JulianoGianlupi]

Hello Maciej,

FPP's example on custom energy formula is

`

<LinkConstituentLaw>
    <!--The following variables lare defined by default: Lambda,Length,TargetLength-->

    <Variable Name='LambdaExtra' Value='1.0'/>
    <Formula>LambdaExtra*Lambda*(Length-TargetLength)^2</Formula>

</LinkConstituentLaw>

`

But that trows an error with the <Formula> tag, player says it should be <Expression>