docs: fix missing closing quote (#4949)

With the current example build fails with:

> notarize: macos: template: failed to apply "{{ isEnvSet
\"MACOS_SIGN_P12 }}": unterminated quoted string

www/docs/customization/notarize.md

@@ -31,7 +31,7 @@ notarize:
       #
       # Default: false
       # Templates: allowed
-      enabled: '{{ isEnvSet "MACOS_SIGN_P12 }}'
+      enabled: '{{ isEnvSet "MACOS_SIGN_P12" }}'
 
       # IDs to use to filter the built binaries.
       #