Minor doc fix (bivariate function registration)

[jbcaillau]

@sshin23 shouldn't the function below depend on both x and y?

ExaModels.jl/src/register.jl

Line 62 in 16a2443

julia> relu23(x) = (x > 0 || y > 0) ? (x + y)^3 : zero(x)

julia> using ExaModels

julia> relu23(x) = (x > 0 || y > 0) ? (x + y)^3 : zero(x)
relu23 (generic function with 1 method)

julia> drelu231(x) = (x > 0 || y > 0) ? 3 * (x + y)^2 : zero(x)
drelu231 (generic function with 1 method)

julia> drelu232(x) = (x > 0 || y > 0) ? 3 * (x + y)^2  : zero(x)
drelu232 (generic function with 1 method)

julia> ddrelu2311(x) = (x > 0 || y > 0) ? 6 * (x + y) : zero(x)
ddrelu2311 (generic function with 1 method)

julia> ddrelu2312(x) = (x > 0 || y > 0) ? 6 * (x + y) : zero(x)
ddrelu2312 (generic function with 1 method)

julia> ddrelu2322(x) = (x > 0 || y > 0) ? 6 * (x + y) : zero(x)
ddrelu2322 (generic function with 1 method)

julia> @register_bivariate(relu23, drelu231, drelu232, ddrelu2311, ddrelu2312, ddrelu2322)

[sshin23]

Thanks, @jbcaillau! It is fixed in #175