doc: cosmetic change to snapshot testing documentation (cloudnative-pg#899)

Pass the URL of the manifest instead of using `curl` in the example
included in the contribution guide.

Closes cloudnative-pg#898

Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>

Author: gbartolini

contribute/README.md

@@ -54,9 +54,8 @@ If you want to test or evaluate the latest development snapshot of
 CloudNativePG before the next official patch release, you can simply run:
 
 ```sh
-curl -sSfL \
-  https://raw.githubusercontent.com/cloudnative-pg/artifacts/main/manifests/operator-manifest.yaml | \
-  kubectl apply -f -
+kubectl apply -f \
+  https://raw.githubusercontent.com/cloudnative-pg/artifacts/main/manifests/operator-manifest.yaml
 ```
 
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