Solargraph uses stdio instead of --stdio (#642)

You can see this in the [default config](https://github.com/neovim/nvim-lspconfig/blob/master/lua/lspconfig/solargraph.lua).

ftplugin/ruby.lua

@@ -1,6 +1,6 @@
 -- If you are using rvm, make sure to change below configuration
 require'lspconfig'.solargraph.setup {
-    cmd = {DATA_PATH .. "/lspinstall/ruby/solargraph/solargraph", "--stdio"},
+    cmd = {DATA_PATH .. "/lspinstall/ruby/solargraph/solargraph", "stdio"},
     on_attach = require'lsp'.common_on_attach,
     handlers = {
         ["textDocument/publishDiagnostics"] = vim.lsp.with(