Fix Incorrect Key for Virtual Text Spacing in Diagnostics Configuration

[eitamal]

This PR corrects the key used to configure the virtual text spacing for diagnostics.

As per Neovim's documentation, the appropriate key for controlling the virtual text's spacing is spacing. However, the current configuration incorrectly uses space.

Before:

virtual_text = { space = 0, prefix = '■' },

After:

virtual_text = { spacing = 0, prefix = '■' },

It's worth noting that this change may not have a noticeable impact on functionality. This is because a nil spacing value might be interpreted similarly to a spacing value of 0.

[ray-x]

Thanks!