Update docs/index.rst

Co-authored-by: Josh Moore <[email protected]>

Author: rabernat

docs/index.rst

@@ -4,7 +4,7 @@
 Zarr
 ====
 
-Zarr is format for the storage of chunked, compressed, N-dimensional arrays.
+Zarr is a format for the storage of chunked, compressed, N-dimensional arrays.
 These documents describe the Zarr format and its Python implementation.
 
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