taskell: Use language-specific heredocs

Author: issyl0

Formula/t/taskell.rb

@@ -43,12 +43,12 @@ def install
   end
 
   test do
-    (testpath/"test.md").write <<~EOS
+    (testpath/"test.md").write <<~MARKDOWN
       ## To Do
 
       - A thing
       - Another thing
-    EOS
+    MARKDOWN
 
     expected = <<~EOS
       test.md