runtime(doc): grammar fixes in doc (#13164)

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/version9.txt b/runtime/doc/version9.txt
index ee5b68d..09d6b3b 100644
--- a/runtime/doc/version9.txt
+++ b/runtime/doc/version9.txt
@@ -1198,7 +1198,7 @@
 Files:      src/evalwindow.c, src/testdir/test_getvar.vim
 
 Patch 8.2.0118
-Problem:    Crash when cycling to buffers involving popup window .
+Problem:    Crash when cycling to buffers involving popup window.
 Solution:   Do not decrement buffer reference count.
 Files:      src/popupwin.c, src/testdir/test_popupwin.vim,
             src/testdir/dumps/Test_popupwin_infopopup_7.dump