fix small word dupe typos

compiler/rustc_error_codes/src/error_codes/E0591.md

@@ -53,8 +53,8 @@ unsafe {
 ```
 
 Here, transmute is being used to convert the types of the fn arguments.
-This pattern is incorrect because, because the type of `foo` is a function
-**item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
+This pattern is incorrect because the type of `foo` is a function **item**
+(`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
 is a function pointer, which is not zero-sized.
 This pattern should be rewritten. There are a few possible ways to do this: