Fix typo in dom/README.md

According to the context, [O] should be [Q].

Bug: None
Change-Id: I76656315f8f388bebb794886974cb4cc08cf0e08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1624145Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Jinsong Fan <fanjinsong@sogou-inc.com>
Cr-Commit-Position: refs/heads/master@{#662103}

third_party/blink/renderer/core/dom/README.md

@@ -785,7 +785,7 @@ Slot Assignments:
 | ------ | --------------------- |
 | slot-G | [H]                   |
 | slot-O | [slot-P]              |
-| slot-P | [O]                   |
+| slot-P | [Q]                   |
 | slot-R | [slot-S]              |
 | slot-S | [T]                   |