patch 8.2.1961: various comments can be improved

Problem:    Various comments can be improved.
Solution:   Various comment adjustments.
diff --git a/src/testdir/shared.vim b/src/testdir/shared.vim
index c7dd41f..3c6fe1c 100644
--- a/src/testdir/shared.vim
+++ b/src/testdir/shared.vim
@@ -223,7 +223,7 @@
   call feedkeys('x', 'nt')
 endfunc
 
-" Get $VIMPROG to run Vim executable.
+" Get $VIMPROG to run the Vim executable.
 " The Makefile writes it as the first line in the "vimcmd" file.
 func GetVimProg()
   if !filereadable('vimcmd')