fix: non-substantive typos (#434)

Co-authored-by: Viktor Szépe <viktor@szepe.net>
Co-authored-by: Edward Z. Yang <ezyang@mit.edu>

tests/HTMLPurifier/ConfigTest.php

@@ -184,7 +184,7 @@ class HTMLPurifier_ConfigTest extends HTMLPurifier_Harness
             )
         );
 
-        // grab a non-existant namespace
+        // grab a non-existent namespace
         $this->expectError('Cannot retrieve undefined namespace Constants');
         $config->getBatch('Constants');