Picture of animal in PR should really be optional

Because it's only folklore and should not be a blocker.

Signed-off-by: Yves Brissaud <yves.brissaud@docker.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -26,5 +26,5 @@ pull request for inclusion in the changelog:
 -->
 
 
-**- A picture of a cute animal (not mandatory but encouraged)**
+**- A picture of a cute animal (not mandatory)**