Replace specific (and outdated) tag with <tag>

It is hard to keep remembering to update the tag, so better use a
placeholder.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

.github/RELEASE_NOTES.template.md

@@ -13,7 +13,7 @@
 All upgrading should be done via the migration script or regenerating the templates.
 
 ```bash
-curl -sSL https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config-python/v0.12/cookiecutter/migrate.py | python3
+curl -sSL https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config-python/<tag>/cookiecutter/migrate.py | python3
 ```
 
 But you might still need to adapt your code:

RELEASE_NOTES.md

@@ -13,7 +13,7 @@
 All upgrading should be done via the migration script or regenerating the templates.
 
 ```bash
-curl -sSL https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config-python/v0.12/cookiecutter/migrate.py | python3
+curl -sSL https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config-python/<tag>/cookiecutter/migrate.py | python3
 ```
 
 But you might still need to adapt your code: