Update pydap link in installation guide (#10885)

The old link doesn't work anymore.

Author: nicrie

doc/getting-started-guide/installing.rst

@@ -28,7 +28,7 @@ For netCDF and IO
 - `netCDF4 <https://github.com/Unidata/netcdf4-python>`__: recommended if you
   want to use xarray for reading or writing netCDF files
 - `scipy <https://scipy.org>`__: used as a fallback for reading/writing netCDF3
-- `pydap <https://www.pydap.org>`__: used as a fallback for accessing OPeNDAP
+- `pydap <https://pydap.github.io/pydap/en/intro.html>`__: used as a fallback for accessing OPeNDAP
 - `h5netcdf <https://github.com/h5netcdf/h5netcdf>`__: an alternative library for
   reading and writing netCDF4 files that does not use the netCDF-C libraries
 - `zarr <https://zarr.readthedocs.io>`__: for chunked, compressed, N-dimensional arrays.