Use isn't instead of shouldn't

This definitely isn't possible, change the phrase.

Author: tcharding

src/context.rs

@@ -61,7 +61,7 @@ pub mod global {
 
 
 /// A trait for all kinds of contexts that lets you define the exact flags and a function to
-/// deallocate memory. It shouldn't be possible to implement this for types outside this crate.
+/// deallocate memory. It isn't possible to implement this for types outside this crate.
 pub unsafe trait Context : private::Sealed {
     /// Flags for the ffi.
     const FLAGS: c_uint;