fix: change default for g:CorpusPreviewWinhighlight

As suggested in:

    #75 (comment)

because:

> Just a nit, but I'd lean toward a different default. VertSplit is
> decoration so colorschemes are not designed with the expectation it
> it will be used to render text. It seems more likely than almost any
> other syntax group to be set up with very low or no contrast between
> fg and bg.

Switched to LineNr, semi-arbitrarily (there is no guaranteed highlight
group that is going to look good with all colorschemes, but this one
should at least not look invisible).

README.md

@@ -213,7 +213,7 @@ let g:CorpusChooserSelectionHighlight='Error'
 Specifies the <strong>`'winhighlight'`</strong> string for the preview pane (the right-hand pane). The default is:
 
 ```
-EndOfBuffer:VertSplit,FoldColumn:StatusLine,Normal:VertSplit".
+EndOfBuffer:LineNr,FoldColumn:StatusLine,Normal:LineNr.
 ```
 
 To override, provide an alternate setting in your <strong>`init.vim`</strong>:

doc/corpus.txt

@@ -236,7 +236,7 @@ chooser listing (the left-hand pane). To override from the default value of
 Specifies the |'winhighlight'| string for the preview pane (the right-hand
 pane). The default is:
 >
-    EndOfBuffer:VertSplit,FoldColumn:StatusLine,Normal:VertSplit".
+    EndOfBuffer:LineNr,FoldColumn:StatusLine,Normal:LineNr.
 <
 To override, provide an alternate setting in your |init.vim|:
 >

lua/corpus/init.lua

@@ -429,7 +429,7 @@ corpus = {
         'winhighlight',
         get_var(
           'CorpusPreviewWinhighlight',
-          'EndOfBuffer:VertSplit,FoldColumn:StatusLine,Normal:VertSplit'
+          'EndOfBuffer:LineNr,FoldColumn:StatusLine,Normal:LineNr'
         )
       )
       vim.api.nvim_win_set_option(

plugin/corpus.vim

@@ -266,7 +266,7 @@
 " pane). The default is:
 "
 " ```
-" EndOfBuffer:VertSplit,FoldColumn:StatusLine,Normal:VertSplit".
+" EndOfBuffer:LineNr,FoldColumn:StatusLine,Normal:LineNr.
 " ```
 "
 " To override, provide an alternate setting in your |init.vim|: