Clean up E0668 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0668.md

@@ -1,11 +1,7 @@
 Malformed inline assembly rejected by LLVM.
 
-LLVM checks the validity of the constraints and the assembly string passed to
-it. This error implies that LLVM seems something wrong with the inline
-assembly call.
+Erroneous code example:
 
-In particular, it can happen if you forgot the closing bracket of a register
-constraint (see issue #51430):
 ```compile_fail,E0668
 #![feature(llvm_asm)]
 
@@ -17,3 +13,10 @@ fn main() {
     }
 }
 ```
+
+LLVM checks the validity of the constraints and the assembly string passed to
+it. This error implies that LLVM seems something wrong with the inline
+assembly call.
+
+In particular, it can happen if you forgot the closing bracket of a register
+constraint (see issue #51430), like in the previous code example.