Hmm. A quick search with EasyFind (brilliant search app for OSX) finds it in post-template.php in wp-includes. You could edit that, but it will get overwritten if you update.
Alternatively, yesterday, I discovered the plugin “Say What?“, which replaces text strings with your replacements on the fly. I found it via this page from the author.
I’ve used it on my English site to change the 404 messages for instance. (On my Italian site, I will be using the language functionality to change what I want in future, but on the English site I didn’t want to add the language files.) The plugin is incredibly simple to use and works well. The author has written 17 plugins (some very complex), so I’m trusting that it will hold up well.
It asks for a “domain” in which each string works. In the case of Customizr text, the domain is “customizr” (at least, that’s what I tried, and it works). I’m not sure what the “domain” would be in the case of wp-includes (“wp-includes”, maybe?). If you try it, let us know how you get on.