docs: add Buffers entry to Nerd Fonts v3 source selector example

[NReilingh]

I recently reconfigured my terminal and editor, so I'm using v3 nerd fonts. Using the winbar source selector, I noticed icons were broken, so I applied the config suggested in the readme. I realized that the buffers entry was missing, so I determined that the existing code point was 0xf719, nf-mdi-file_document_box:

neo-tree.nvim/lua/neo-tree/sources/buffers/init.lua

Line 14 in 7f6fa04

display_name = "  Buffers "

The equivalent glyph in v3 has a different name, nf-md-text_box, and is at 0xf021a.

This PR adds a line to the readme config example to fix the buffer icon for V3 nerd fonts.

[NReilingh]

@cseickel The reason I filed this against branch v2.x is because that's what is currently displayed on the main project page (https://github.com/nvim-neo-tree/neo-tree.nvim), whereas it's kind of useless on main since this already has support for V3 nerd fonts, so the workaround in the README can be removed.