Edit "java_language_server" description in CONFIG.md

[drybalka]

First of all, I am quite new to lspconfig. I was trying to start a java_language_server similarly to pyright, so I went to CONFIG.md and there it says (very similarly to pyright):
Snippet to enable the language server:

require'lspconfig'.java_language_server.setup{}

Well, when I put this string as-is into my init.lua nothing works. Moreover, this even breaks :LspInfo command with a very cryptic error message "E5108: Error executing lua Vim:E928: String required". Now of course after a bit of self-reflection I found that I have to specify cmd pointing to jls, which is apparently the missing string. I propose to implement one or more of the following 3 fixes:

1. Change the snippet example in CONFIG.md to

require'lspconfig'.java_language_server.setup {
  cmd = { "path/to/your/lang_server_linux.sh"}
}

or to something that would work automatically with a package-installed language server:

require'lspconfig'.java_language_server.setup {
  cmd = {"java-language-server"}
}

2. Change the error message from LspInfo (and wherever else) from the cryptic E5108 to something more concrete, like "set cmd variable in java_language_server.setup{}" or "cmd variable for java_language_server not defined".
3. A bit more controversially one can change the default value of cmd from {} to { "path/to/your/lang_server_linux.sh"} or to {"java-language-server"}. At least this way you can get a more adequate error from :LspInfo.

[mjlbach]

These are all community contributed, if you'd like to change the java_language_server documentation, you can make a pull request. FWIW, the jdtls documentation makes this more clear, so you can use that as a template.