Work around the last problems reported by the linter

This works around these problems:

README.md:191:3:   https://matrix.to/#/#ada-lang_Lobby:gitter.im  remark-lint:double-link
README.md:199:112: https://matrix.to/#/#ada-lang:matrix.org       remark-lint:double-link

Which are due to an issue in the linter: Scrum/remark-lint-double-link#10

Closes #107

README.md

@@ -196,7 +196,9 @@ A curated list of awesome resources related to the Ada and SPARK programming lan
 - [acm-sig-ada](https://www.sigada.org/) - The Special Interest Group on Ada.
 - [ada-resource-association](https://www.adaic.org/community/) - Since 1990 the Ada Resource Association's principal mission has been “To ensure continued success of Ada users and promote Ada use in the software industry”.
 - [ada-information-clearinghouse](https://www.adaic.org/) - News and resources for the Ada programming language.
-- [ada-planet](https://www.laeran.pl/adaplanet/i/) - News Aggregator from the Ada programming language world. ([RSS feed via Matrix](https://matrix.to/#/#ada-lang:matrix.org))
+- [ada-planet](https://www.laeran.pl/adaplanet/i/) - News Aggregator from the Ada programming language world. ([RSS feed via Matrix][ada-planet-via-matrix])
+
+[ada-planet-via-matrix]: https://matrix.to/#/#ada-lang:matrix.org
 
 ### Competition
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