fix typo

Author: coke

doc/Language/performance.pod6

@@ -52,7 +52,7 @@ be slow to open in a browser. In that case, if you use the C<--profile-filename=
 an extension of C<.json>, you can use the L<Qt viewer|https://github.com/tadzik/p6profiler-qt> on the resulting
 JSON file.
 
-Another option (especially useful for profiles too big even for the Qt viewer) is to use an extention of C<.sql>.
+Another option (especially useful for profiles too big even for the Qt viewer) is to use an extension of C<.sql>.
 This will write the profile data as a series of SQL statements, suitable for opening in SQLite.
 
     # create a profile