Correct formula in documentation of Solver::Options::function_tolerance.
Correct formula in documentation of Solver::Options::function_tolerance.

Change-Id: I5cfb9d58bd2ea12e9be46fb9d9e5a37466b0a7cb
kartikkumar/ceres-solver,kartikkumar/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,stonier/ceres_solver,entylop/ceres-solver,kartikkumar/ceres-solver,entylop/ceres-solver,DougFirErickson/ceres-solver,timfel/ceres-bin,whuaegeanse/ceres-solver,stonier/ceres_solver,timfel/ceres-bin,stonier/ceres_solver,kartikkumar/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,ysonggit/ceres-solver,timfel/ceres-bin,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,timfel/ceres-bin,stonier/ceres_solver,ysonggit/ceres-solver,whuaegeanse/ceres-solver