Ppdate packer settings for Semshi

If Semshi is set to opt plugin, when we run PackerUpdate, the command `:
UpdateRemotePlugins` will be run. At this time, Semshi is not in the
runtimepath, so its manifest will not be generated in rplugin.vim
(located under ~/.local/share/nvim/rplugin.vim). So when we open a
Python file afterwards, although Semshi is in runtimepath, but its
manifest is missing, so we see error that command `Semshi enable` is not
found.

So after loading this plugin, i.e., when we open a Python source file,
we need to run UpdateRemotePlugins so that semshi can work correctly.

lua/plugins.lua

@@ -34,7 +34,7 @@ require('packer').startup(
 
     -- Python syntax highlighting and more
     if (vim.g.is_mac == 1) or (vim.g.is_win == 1) then
-      use {'numirias/semshi', run = ':UpdateRemotePlugins', ft = 'python'}
+      use {'numirias/semshi', ft = 'python', config = 'vim.cmd [[UpdateRemotePlugins]]'}
     end
 
     -- Python indent (follows the PEP8 style)