fix heading nesting

CONTRIBUTING.md

@@ -16,5 +16,5 @@
 
 * In your pull request, do not add documentation or re-build the minified `underscore-umd-min.js` file. We'll do those things before cutting a new release.
 
-## "Help, cloning fails with `fatal: fsck error in packed object`"
+### "Help, cloning fails with `fatal: fsck error in packed object`"
 This error is caused by zero-padded file modes in the commit history. As fixing this is highly destructive, we suggest ignoring these warnings. The simplest way is to instruct git to do so when cloning. For example, to clone from `jashkenas/underscore`, run the following command: `git clone --config transfer.fsckobjects=false git@github.com:jashkenas/underscore.git`. If cloning from a different user or organization, replace `jashkenas` with their name in the previous command.