[SPARK-16035][PYSPARK] Fix SparseVector parser assertion for end parenthesis

## What changes were proposed in this pull request?
The check on the end parenthesis of the expression to parse was using the wrong variable. I corrected that.
## How was this patch tested?
Manual test

Author: andreapasqua <andrea@radius.com>

Closes #13750 from andreapasqua/sparse-vector-parser-assertion-fix.