CLN: fix typo in ctors.SeriesDtypesConstructors setup
Our benchmark for creating an `Index` from an array of floats does so from a **2-d** array, seemingly dating from when this benchmark tested `DataFrame` creation (see https://github.com/pandas-dev/pandas/commit/04db779d4c93d286bb0ab87780a85d50ec490266#diff-408a9d5de471447a365854703ca1856dL22).

Changing this to a 1-d array has no significant impact on benchmark times while reducing setup by ~40 seconds.

Before:
```
$ time asv dev -b ctors.SeriesDtypes
· Discovering benchmarks
· Running 4 total benchmarks (1 commits * 1 environments * 4 benchmarks)
[  0.00%] ·· Benchmarking existing-py_home_chris_anaconda3_bin_python
[ 12.50%] ··· ctors.SeriesDtypesConstructors.time_dtindex_from_index_with_series        2.44±0ms
[ 25.00%] ··· ctors.SeriesDtypesConstructors.time_dtindex_from_series                   2.25±0ms
[ 37.50%] ··· ctors.SeriesDtypesConstructors.time_index_from_array_floats                760±0μs
[ 50.00%] ··· ctors.SeriesDtypesConstructors.time_index_from_array_string                942±0μs

real    0m51.040s
user    0m39.813s
sys     0m10.172s
```

After:
```
$ time asv dev -b ctors.SeriesDtypes
· Discovering benchmarks
· Running 4 total benchmarks (1 commits * 1 environments * 4 benchmarks)
[  0.00%] ·· Benchmarking existing-py_home_chris_anaconda3_bin_python
[ 12.50%] ··· ctors.SeriesDtypesConstructors.time_dtindex_from_index_with_series        2.37±0ms
[ 25.00%] ··· ctors.SeriesDtypesConstructors.time_dtindex_from_series                   2.33±0ms
[ 37.50%] ··· ctors.SeriesDtypesConstructors.time_index_from_array_floats                763±0μs
[ 50.00%] ··· ctors.SeriesDtypesConstructors.time_index_from_array_string                885±0μs

real    0m13.000s
user    0m9.031s
sys     0m3.547s
```

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
