Fix Word2Vec benchmark claim with accurate cited numbers

Changed unsourced 'r ≈ 0.7' claim to accurate values with citations:
- Relatedness (WordSim-353, MEN): ρ ≈ 0.70–0.75 (Pennington et al., 2014)
- Strict similarity (SimLex-999): ρ ≈ 0.44 (Hill et al., 2015)

Updated variance explanation accordingly (~20% not ~50%)

Author: jeremymanning