fix(docs): duplicate tag, wrong formatting

Fixes build error:
    cd /usr/home/build/neovim/build/runtime && /usr/local/bin/cmake -E remove doc/* && /usr/local/bin/cmake -E copy_directory /usr/home/build/neovim/runtime/doc doc && /usr/home/build/neovim/build/bin/nvim -u NONE -i NONE -e --headless -c helptags\ ++t\ doc -c quit
    Error detected while processing command line:
    E154: Duplicate tag "vim.register_keystroke_callback()" in file doc/lua.txt

src/nvim/api/vim.c

@@ -1358,7 +1358,7 @@ void nvim_set_current_tabpage(Tabpage tabpage, Error *err)
   }
 }
 
-/// Creates a new *namespace*, or gets an existing one.
+/// Creates a new \*namespace\*, or gets an existing one.
 ///
 /// Namespaces are used for buffer highlights and virtual text, see
 /// |nvim_buf_add_highlight()| and |nvim_buf_set_extmark()|.

src/nvim/lua/vim.lua

@@ -433,6 +433,7 @@ function vim.notify(msg, log_level, _opts)
 end
 
 
+---@private
 function vim.register_keystroke_callback()
   error('vim.register_keystroke_callback is deprecated, instead use: vim.on_key')
 end