Update distro prereq links in README

[slel]

In #29124 the text files describing prerequisite package
names for each distribution moved from build/pkgs/*.txt
to build/pkgs/_prereq/distros/*.txt.

This ticket updates links in the README accordingly.

The README for the latest development version of Sage
can always be found at:

• https://github.com/sagemath/sage-prod/blob/develop/README.md

CC: @mkoeppe @slel

Component: build

Author: Samuel Lelièvre

Branch: bd9a33c

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/31909

[slel]

New commits:

bd9a33c

31909: Update distro prereq links in README

[slel]

Commit: bd9a33c

[slel]

Branch: public/31909

[mkoeppe]

Reviewer: Matthias Koeppe

[mkoeppe]

comment:2

Thanks for catching this!

[slel]

comment:3

Thanks for the review!

[vbraun]

Changed branch from public/31909 to bd9a33c

[slel]

Changed commit from bd9a33c to none

[slel]

comment:5

Add link to current develop README in ticket description.

Since this ticket was merged in Sage 9.4.beta4,
that now has the correct links.