Fix test involving JSON gem changes

Signed-off-by: Alexandre Terrasa <alexandre.terrasa@shopify.com>

Author: Morriar

gem/test/rbi-central/repo_test.rb

@@ -17,7 +17,7 @@ def test_index_bad_json
       e = assert_raises(RBICentral::Index::Error) do
         @repo.index
       end
-      assert_equal("Invalid JSON in `index.json`: unexpected token at ''", e.message)
+      assert_equal("Invalid JSON in `index.json`: unexpected end of input", e.message)
     end
 
     def test_index_bad_schema