build: rename rockspec to standard luarocks format

Problem: rockspec file was named 'rockspec' instead of the
luarocks-required '<package>-<version>.rockspec' format.

Solution: rename to todo.nvim-scm-1.rockspec.

rockspec

@@ -1,30 +0,0 @@
-rockspec_format = '3.0'
-package = 'todo.nvim'
-version = 'scm-1'
-
-source = {
-  url = 'git+https://github.com/barrettruth/todo.nvim.git',
-}
-
-description = {
-  summary = 'Oil-like task management for Neovim',
-  homepage = 'https://github.com/barrettruth/todo.nvim',
-  license = 'MIT',
-}
-
-dependencies = {
-  'lua >= 5.1',
-}
-
-test_dependencies = {
-  'nlua',
-  'busted >= 2.1.1',
-}
-
-test = {
-  type = 'busted',
-}
-
-build = {
-  type = 'builtin',
-}