docs: use k
as constant name instead of a
in simpleLinearRegression
Function uses `k` internally.
E.g.
```sql
SELECT arrayReduce('simpleLinearRegression', [0, 1, 2, 3], [0, 1, 2, 3]) FORMAT JsonEachRow
```
outputs `{"arrayReduce('simpleLinearRegression', [0, 1, 2, 3], [0, 1, 2, 3])":{"k":1,"b":0}}`