Rollup merge of #73057 - GuillaumeGomez:cleanup-e0644, r=Dylan-DPC

Clean up E0644 explanation

r? @Dylan-DPC

src/librustc_error_codes/error_codes/E0644.md

@@ -1,6 +1,6 @@
 A closure or generator was constructed that references its own type.
 
-Erroneous example:
+Erroneous code example:
 
 ```compile_fail,E0644
 fn fix<F>(f: &F)