[SPARK-9198] [MLLIB] [PYTHON] Fixed typo in pyspark sparsevector doc tests

Several places in the PySpark SparseVector docs have one defined as:
```
SparseVector(4, [2, 4], [1.0, 2.0])
```
The index 4 goes out of bounds (but this is not checked).

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #7541 from jkbradley/sparsevec-doc-typo-fix and squashes the following commits:

c806a65 [Joseph K. Bradley] fixed doc test
e2dcb23 [Joseph K. Bradley] Fixed typo in pyspark sparsevector doc tests