[pitivi] Update Testing.md

[Thibault Saunier <tsaunier src gnome org>]

commit 37e8b90281ba9968a745024a88a21dc1f863037d
Author: Janish <jsjanish 1999 gmail com>
Date:   Fri Mar 15 15:15:50 2019 +0000

    Update Testing.md

 docs/Testing.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/docs/Testing.md b/docs/Testing.md
index e1927223..71a67677 100644
--- a/docs/Testing.md
+++ b/docs/Testing.md
@@ -1,6 +1,6 @@
 # Testing
 
-We have tree sets of tests:
+We have three sets of tests:
  - the normal unit tests (ninja test)
  - integration tests using GstValidate.
  - [manual tests](QA_Scenarios.md)