chore(towncrier): indent (PR ref line by two spaces in template

So the rendered changelog has the (PR [...]) line aligned as a list
continuation under its bullet. Verified with both short and wrapped
entries via `towncrier build --draft`.

changelog/_template.md.j2

@@ -5,7 +5,7 @@
 
 {% for text, values in sections[section][category].items() %}
 {{ text }}
-(PR {{ values|join(', ') }})
+  (PR {{ values|join(', ') }})
 
 {% endfor %}
 {% endfor %}