fix: correct doctest example for tree sync (closes #8310)

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Author: sujal sunil chaudhary

lib/node_modules/@stdlib/_tools/pkgs/tree/lib/sync.js

@@ -42,6 +42,8 @@ var defaults = require( './config.json' );
 * @returns {Object} tree
 *
 * @example
+* var pkgTree = require( '@stdlib/_tools/pkgs/tree/lib/sync' );
+*
 * var tree = pkgTree();
 * // returns {...}
 */