[_fe_analyzer_shared] Fix missing reference in a documentation in string_canonicalizer

Change-Id: I24f93b4461bb9ecb7fcdeccad97cd1ae85ff90bb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392340
Commit-Queue: Ömer Ağacan <[email protected]>
Reviewed-by: Jens Johansen <[email protected]>

Author: osa1

pkg/_fe_analyzer_shared/lib/src/scanner/string_canonicalizer.dart

@@ -141,7 +141,7 @@ class _StringCanonicalizer {
   /// Number of [_StringNode]s in the cache.
   int _stringCount = 0;
 
-  /// Number of []s in the cache.
+  /// Number of [_Utf8Node]s in the cache.
   int _utf8StringCount = 0;
 
   /// Memory consumption of [_StringNode]s.