[MINOR][ML] Fix naming conventions of AFTSurvivalRegression coefficients

Rename ```regressionCoefficients``` back to ```coefficients```, and name ```weights``` to ```parameters```.
See discussion [here](https://github.com/apache/spark/pull/9311/files#diff-e277fd0bc21f825d3196b4551c01fe5fR230). mengxr vectorijk dbtsai

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #9431 from yanboliang/aft-coefficients.