Fix spelling in CONTRIBUTING.

Minor spelling mistake correction.

TBR=thestig@chromium.org

Change-Id: Ie429ef01c7c9108fe295c4a0f8b4cd28d0ba306e
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/70390
Reviewed-by: dsinclair <dsinclair@chromium.org>
Auto-Submit: dsinclair <dsinclair@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 265e1ab..5f2a9f7 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -99,7 +99,7 @@
 order to validate the new additions are available.
 
 ## Trybot Access
-Changes must pass the try bots before the are merged into the repo. For your
+Changes must pass the try bots before they are merged into the repo. For your
 first few CLs the try bots will need to be triggered by a committer. After
 you've submitted 2-3 CLs you can request try bot access by emailing one of the
 OWNERS and requesting try bot access. This will allow you to trigger the bots