src: fix typo string_search.h comment

PR-URL: #21115
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

src/string_search.h

@@ -559,7 +559,7 @@ size_t StringSearch<Char>::InitialSearch(
   return subject.length();
 }
 
-// Perform a a single stand-alone search.
+// Perform a single stand-alone search.
 // If searching multiple times for the same pattern, a search
 // object should be constructed once and the Search function then called
 // for each search.