patch 9.0.0678: using exclamation marks on :function

Problem:    Using exclamation marks on :function.
Solution:   Use :func and :endfunc as usual.
diff --git a/src/testdir/test_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index fb4b945..9998cc5 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -220,7 +220,7 @@
   %bwipe!
 endfunc
 
-function! Test_prompt_while_writing_to_hidden_buffer()
+func Test_prompt_while_writing_to_hidden_buffer()
   call CanTestPromptBuffer()
   CheckUnix