docs: fix broken porting advice for intl dependency in release notes

See https://github.com/theimpossibleastronaut/rmw/pull/345#discussion_r967876201

The check for `.found()` implies the dependency lookup should be
optional, but the example didn't make it optional.

docs/markdown/Release-notes-for-0.59.0.md

@@ -125,7 +125,7 @@ endif
 one may simply use:
 
 ```
-intl_dep = dependency('intl')
+intl_dep = dependency('intl', required: false)
 
 if intl_dep.found()
   # build options that need gettext