fix: update branch in default pr template

Author: bchampp

.github/pull_request_template.md

@@ -5,6 +5,6 @@
 ## Dependencies
 If this PR requires testing against a specific branch of the Python Language SDK (e.g., for unreleased changes), uncomment and specify the branch below. Otherwise, leave commented to use the main branch.
 
-<!-- PYTHON_LANGUAGE_SDK_BRANCH: branch-name -->
+<!-- PYTHON_LANGUAGE_SDK_BRANCH: main -->
 
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