CLN: consistent use of rval within left_join_indexer_unique (minor cleanup)
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Very minor cleanup. Replacing `right[j]` with `rval` to be consistent with code above and below the line in question. (`rval = right[j]`) 
thx @lukemanley 