Documentation fix for E0368

Made the 'good' example compile.

I got to the E0368 error page by following the link in the output of the
compiler. My understanding is that the first example is 'bad' and the
second one is 'good'. Following that logic, I pasted the second example
into the file and to my surprise it did not compile. This commit fixes
the example to make it paste-able.

src/librustc_typeck/diagnostics.rs

@@ -3167,8 +3167,8 @@ x <<= 2;
 To fix this error, please check that this type implements this binary
 operation. Example:
 
-```compile_fail
-let x = 12u32; // the `u32` type does implement the `ShlAssign` trait
+```
+let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait
 
 x <<= 2; // ok!
 ```