docs: Correct grammar issues in a/an usage

[star1327p]

Correct grammar issues in a/an usage

The correct usages are "an np" and "an FSMap", because the phonetic spelling of the word starts with a vowel.
https://www.grammarly.com/blog/grammar/indefinite-articles-a-and-an/

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.90%. Comparing base (34859c4) to head (f42dd31).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/core/metadata/v3.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3634   +/-   ##
=======================================
  Coverage   60.90%   60.90%           
=======================================
  Files          86       86           
  Lines       10174    10174           
=======================================
  Hits         6196     6196           
  Misses       3978     3978           

Files with missing lines	Coverage Δ
src/zarr/core/array.py	68.48% <ø> (ø)
src/zarr/core/buffer/core.py	30.34% <ø> (ø)
src/zarr/core/buffer/cpu.py	41.37% <ø> (ø)
src/zarr/core/buffer/gpu.py	43.37% <ø> (ø)
src/zarr/core/dtype/npy/bytes.py	53.00% <ø> (ø)
src/zarr/core/dtype/npy/int.py	53.58% <ø> (ø)
src/zarr/storage/_common.py	66.00% <ø> (ø)
src/zarr/storage/_fsspec.py	8.67% <ø> (ø)
src/zarr/storage/_utils.py	78.33% <ø> (ø)
src/zarr/testing/buffer.py	100.00% <ø> (ø)
... and 1 more

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[TomAugspurger]

Thanks. One comment then this looks good.