Update README.md

Co-authored-by: David Stansby <[email protected]>

Author: tlambert03

README.md

@@ -9,7 +9,7 @@
 ```sh
 pip install -U pydantic-zarr
 # or, with zarr i/o support
-pip install -U pydantic-zarr[zarr]
+pip install -U "pydantic-zarr[zarr]"
 ```
 
 ## Getting help