vim-patch:63d08d4: runtime(doc): update :h write-plugin and readdir() example

1) The complete example plugin at :h write-plugin the end of the
   documentation didn't reflect the snippets provided during the
   tutorial. In most of the Add() function, interpolated strings are
   used, but the final result used concatenated strings.

2) The example provided in readdir() to get a list of files ending in
   ".txt" didn't properly escape the dot in the regular expression:
      readdir(dirname, {n -> n =~ '.txt$'})
   This would match any file ending with "txt" that is at least 4
   characters long, instead it should be:
      readdir(dirname, {n -> n =~ '\.txt$'})

closes: vim/vim#21123

63d08d4386

Co-authored-by: Josep Puigdemont <josep.puigdemont@gmail.com>

runtime/doc/vimfn.txt

@@ -8277,7 +8277,7 @@ readdir({directory} [, {expr}])                                      *readdir()*
 		Each time {expr} is evaluated |v:val| is set to the entry name.
 		When {expr} is a function the name is passed as the argument.
 		For example, to get a list of files ending in ".txt": >vim
-		  echo readdir(dirname, {n -> n =~ '.txt$'})
+		  echo readdir(dirname, {n -> n =~ '\.txt$'})
 <		To skip hidden and backup files: >vim
 		  echo readdir(dirname, {n -> n !~ '^\.\|\~$'})
 

runtime/lua/vim/_meta/vimfn.gen.lua

@@ -7425,7 +7425,7 @@ function vim.fn.readblob(fname, offset, size) end
 --- Each time {expr} is evaluated |v:val| is set to the entry name.
 --- When {expr} is a function the name is passed as the argument.
 --- For example, to get a list of files ending in ".txt": >vim
----   echo readdir(dirname, {n -> n =~ '.txt$'})
+---   echo readdir(dirname, {n -> n =~ '\.txt$'})
 --- <To skip hidden and backup files: >vim
 ---   echo readdir(dirname, {n -> n !~ '^\.\|\~$'})
 ---

src/nvim/eval.lua

@@ -8920,7 +8920,7 @@ M.funcs = {
       Each time {expr} is evaluated |v:val| is set to the entry name.
       When {expr} is a function the name is passed as the argument.
       For example, to get a list of files ending in ".txt": >vim
-        echo readdir(dirname, {n -> n =~ '.txt$'})
+        echo readdir(dirname, {n -> n =~ '\.txt$'})
       <To skip hidden and backup files: >vim
         echo readdir(dirname, {n -> n !~ '^\.\|\~$'})