Fix typo with custom a custom -> with a custom

compiler/rustc_error_codes/src/error_codes/E0493.md

@@ -21,7 +21,7 @@ static FOO: Foo = Foo { field1: (DropType::A, DropType::A).1 }; // error!
 The problem here is that if the given type or one of its fields implements the
 `Drop` trait, this `Drop` implementation cannot be called within a const
 context since it may run arbitrary, non-const-checked code. To prevent this
-issue, ensure all values with custom a custom `Drop` implementation escape the
+issue, ensure all values with a custom `Drop` implementation escape the
 initializer.
 
 ```