[MINOR][DOC] Unify example marks

## What changes were proposed in this pull request?
1, `**Example**` => `**Examples**`, because more algos use `**Examples**`.
2,  delete `### Examples` in `Isotonic regression`, because it's not that special in http://spark.apache.org/docs/latest/ml-classification-regression.html
3, add missing marks for `LDA` and other algos.

## How was this patch tested?
No tests for it only modify doc

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #15783 from zhengruifeng/doc_fix.