tests/data/README.md: Add a blank line

Add a blank line before the code span (three backticks).
diff --git a/tests/data/README.md b/tests/data/README.md
index 768292f..a02c753 100644
--- a/tests/data/README.md
+++ b/tests/data/README.md
@@ -206,6 +206,7 @@
 correctly (i.e.) it should report them as files with alpha channel.
 
 Box structure of the items in this file:
+
 ```
 [primary item grid]
    ^       ^