Make the dependencies part explicit (#269)

Clarify that the recommendation is about "how long new releases of a
package should permit older versions of dependencies", not the more
conventional meaning of support as "we fix bugs for old versions" — like
most medium-sized open-source projects, we (xarray) only release fixes
for the current version of xarray

ref
pydata/xarray#8237 (reply in thread)

@andersy005 @shoyer @dcherian (added by @jarrodmillman )

---------

Co-authored-by: Jarrod Millman <[email protected]>

spec-0000/index.md

@@ -29,8 +29,8 @@ All versions refer to feature releases (i.e., Python 3.8.0, NumPy 1.19.0; not Py
 
 Specifically, we recommend that:
 
-1. Support for a given version of Python be dropped **3 years** after its initial release.
-2. Support for a given version of other core packages be dropped **2 years** after their initial release.
+1. Support for Python versions be dropped **3 years** after their initial release.
+2. Support for core package dependencies be dropped **2 years** after their initial release.
 
 ### Core Project Endorsement