README: fix Devhelp url

Imendio is long gone, and the old URL was returning a 404.
3 jobs from wip/aleksander/fix-devhelp-url in 13 minutes and 33 seconds
Status Job ID Name Coverage
  Build
passed #212775
autotools-build

00:03:05

passed #212776
meson-build

00:05:11

 
  Test
passed #212777
autotools-test

00:08:21

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