runtime(doc): Fix notation of "Vim script" and "Vim9 script"

closes: #17213

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/vim9.vim b/src/testdir/vim9.vim
index 64922b7..b994855 100644
--- a/src/testdir/vim9.vim
+++ b/src/testdir/vim9.vim
@@ -1,6 +1,6 @@
 vim9script
 
-# Utility functions for testing vim9 script
+# Utility functions for testing Vim9 script
 
 # Use a different file name for each run.
 var sequence = 1