I suspect you have trained a series of RF regression models and have plotted explained variance(not error) against training set size. Explained variance is the opposite than a error. The value would be between 0 and 1.
Secondly it does not make much sense to diagnose training explained accuracy for a random forest. Samples take the same paths through the trees when training and predicting, so of course a near perfect fit is obtained. That is why out-of-bag training accuracy/error is used.
The cross-validated score increases a little because more samples both lowers bias(deeper trees + denser sampling from data structure) and lowers variance(decreased tree correlation + less sample error).
So everything looks ok and you probably neither have overly over- nor underfitting. I would prefer to (a) simply plot OOB-CV against different settings of hyperparameters or (b) wrap the model in a repeated nested-CV grid search, if you wanna be really thorough. You will probably find the default parameters are close to optimal.