This is https://github.com/Elgg/Elgg/blob/3.3/mod/uservalidationbyemail/languages/en.php#L12
Of course, you can edit this file -> save it -> run 'Flush the caches'.
But you will lost all changes in next Elgg update.
So, I very recommend to use this awesome plugin to edit any language strings which you want.
Please, read these docs (especcially about vsprintf syntax) to don't make fatal errors on your site.
I've made a pull request with a fix at https://github.com/Elgg/Elgg/pull/13252. If this PR gets merged the error will be fixed in the next release, i.e. Elgg 3.3.8.
In the meantime, you can correct the string yourself as RvR suggested by editing vendor/elgg/elgg/mod/uservalidationbyemail/languages/en.php (or if you installed Elgg with the zip file edit mod/uservalidationbyemail/languages/en.php). If the wrong string is fixed in the next Elgg release you would have to edit the file only this one time now.
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