minor #13691 Fix yaml spec link (Cartman34)

This PR was submitted for the 5.0 branch but it was merged into the 3.4 branch instead (closes #13691).

Discussion
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Fix yaml spec link

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Commits
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72287a2 Fix yaml spec link

Author: xabbuh

components/yaml/yaml_format.rst

@@ -339,4 +339,4 @@ The following YAML features are not supported by the Symfony Yaml component:
 
 .. _`ISO-8601`: http://www.iso.org/iso/iso8601
 .. _`YAML website`: http://yaml.org/
-.. _`YAML specification`: http://www.yaml.org/spec/1.2/spec.html
+.. _`YAML specification`: https://yaml.org/spec/1.2/spec.html