Fix incorrect reference to VersionStrategy in README (#40)

Author: phpfour

README.md

@@ -158,8 +158,8 @@ Post::withoutVersion(function () use ($post) {
 
 You can set the following different version policies through property `protected $versionStrategy`:
 
--   `Overtrue\LaravelVersionable::DIFF` - Version content will only contain changed attributes (Default Strategy).
--   `Overtrue\LaravelVersionable::SNAPSHOT` - Version content will contain all versionable attributes values.
+-   `Overtrue\LaravelVersionable\VersionStrategy::DIFF` - Version content will only contain changed attributes (default strategy).
+-   `Overtrue\LaravelVersionable\VersionStrategy::SNAPSHOT` - Version content will contain all versionable attributes values.
 
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