Update docs/source/installation.rst

Co-authored-by: Sadie L. Bartholomew <sadie.bartholomew@ncas.ac.uk>

Author: davidhassell

docs/source/installation.rst

@@ -301,7 +301,7 @@ environments for which these features are not required.
 
 * `healpix <https://pypi.org/project/healpix>`_, version 2025.1 or
   newer. This package is not required to read and write HEALPix
-  datasets, but may be needed for particular manipulations with
+  datasets, but is needed for particular manipulations with
   HEALPix grids, such as creating latitude and longitude coordinates,
   regridding, some changes to the refinement level, and some
   collapses.