Switch Chromium references to main branch.

Chromium switched to the main branch last week, so update references to
point to that branch.

Change-Id: I7c3699a7c0aa387fbed8dc5ba86487018ff672bc
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/79250
Commit-Queue: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Auto-Submit: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
diff --git a/docs/getting-started.md b/docs/getting-started.md
index 6077d21..e5b261b 100644
--- a/docs/getting-started.md
+++ b/docs/getting-started.md
@@ -181,7 +181,7 @@
 
 
 
-[chrome-plugin]: https://chromium.googlesource.com/chromium/src/+/master/pdf/
+[chrome-plugin]: https://chromium.googlesource.com/chromium/src/+/main/pdf/
 [pdfium-public]: https://pdfium.googlesource.com/pdfium/+/master/public/
 [pdfium-v8]: /docs/v8-getting-started.md
 [pdfium-edit-guide]: /docs/pdfium-edit-guide.md