Fix the Markdown Lint configuration

It would have been nice to keep the indicator as to the real reason for
those warnings, "MD013" really does not tell anybody anything.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.markdownlint.json

@@ -1,4 +1,4 @@
 {
-    "MD013/line-length": false,
+    "MD013": false,
     "MD014": false
 }