org: update plantuml download URL

* doc/Makefile.am: Here.
10 jobs for adl/gitlab-ci in 58 minutes and 3 seconds (queued for 1 second)
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passed arch-clang #1877

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failed debian-gcc-snapshot #1876

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failed debian-stable-gcc #1874

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passed mingw-shared #1879

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passed mingw-static #1883

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failed arch-gcc-glibcxxdebug #1878

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failed debian-unstable-gcc-coverage #1875

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failed mingw-static #1880

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Name Stage Failure
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debian-unstable-gcc-coverage Build
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debian-gcc-snapshot Build
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debian-stable-gcc Build
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