Data on neutron-neutron scattering length extracted in the n+2H-->n+n+p reaction at 60 MeV

${}^1{S}_0$ $\mathit{\text{nn}}$-scattering length has been measured in the $\mathit{\text{nd}}$-breakup reaction at 60 MeV. The experiment was performed in the final state interaction geometry with registration of all three secondary particles. The scattering length $a_{nn}=-16.3\pm 0.5$~fm was obtained from a comparison of the experimental dependence of the $\mathit{\text{nd}}$-breakup yield on the relative energy of $\mathit{\text{nn}}$-pair with the simulation results. An analysis of this value of scattering length together with the data obtained in other experiments on $\mathit{\text{nd}}$- and $\mathit{\text{dd}}$-breakup confirms the hypothesis about the influence of $3N$-forces on the values of $\mathit{\text{nn}}$-interaction parameters extracted in these reactions.