Merge pull request #6020 from jbampton/fix-markdown-link

Enable markdownlint rule MD052

.github/linters/.markdown-lint.yml

@@ -27,6 +27,3 @@ MD040: false
 
 # MD041 first-line-heading/first-line-h1 - First line in a file should be a top-level heading
 MD041: false
-
-# MD052/reference-links-images Reference links and images should use a label that is defined
-MD052: false

doc/guides/gc-arena-howto.md

@@ -3,6 +3,7 @@
 _This is an English translation of [Matz's blog post][matz blog post]
 written in Japanese._
 _Some parts are updated to reflect recent changes._
+
 [matz blog post]: <https://www.rubyist.net/~matz/20130731.html>
 
 When you are extending mruby using C language, you may encounter