[pitivi] docs: Update how to run the integration tests

[Alexandru Băluț <alexbalut src gnome org>]

commit 8f8a84114452a1fcd072eb79473a9b24f2362d03
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Sun Dec 29 01:19:52 2019 +0100

    docs: Update how to run the integration tests

 docs/Testing.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/docs/Testing.md b/docs/Testing.md
index 1be92ba2..c371c145 100644
--- a/docs/Testing.md
+++ b/docs/Testing.md
@@ -69,5 +69,5 @@ each time Pitivi is used.
 You can run the integration tests with:
 
 ```
-$ tests/validate-tests/runtests
+$ ptvenv tests/validate-tests/runtests
 ```