Improve miri's error reporting in check_in_alloc

Co-Authored-By: Ralf Jung <post@ralfj.de>

src/librustc/mir/interpret/allocation.rs

@@ -38,7 +38,7 @@ impl Display for CheckInAllocMsg {
         write!(f, "{}", match *self {
             CheckInAllocMsg::MemoryAccessTest => "Memory access",
             CheckInAllocMsg::NullPointerTest => "Null pointer test",
-            CheckInAllocMsg::PointerArithmeticTest => "Pointer arithmetic test",
+            CheckInAllocMsg::PointerArithmeticTest => "Pointer arithmetic",
             CheckInAllocMsg::InboundsTest => "Inbounds test",
         })
     }