uefi-macros: Fix error tests

The new error text is actually not as good, we'll want to make some updates to
the `entry` macro to improve that in the future.

Author: nicholasbishop

uefi-macros/tests/ui/entry_bad_arg.stderr

@@ -6,3 +6,4 @@ error[E0308]: mismatched types
   |
   = note: expected fn pointer `extern "efiapi" fn(uefi::Handle, uefi::table::SystemTable<uefi::table::Boot>) -> uefi::Status`
                 found fn item `extern "efiapi" fn(uefi::Handle, uefi::table::SystemTable<uefi::table::Boot>, usize) -> uefi::Status {main}`
+  = note: when the arguments and return types match, functions can be coerced to function pointers

uefi-macros/tests/ui/entry_bad_return_type.stderr

@@ -2,7 +2,8 @@ error[E0308]: mismatched types
  --> tests/ui/entry_bad_return_type.rs:8:4
   |
 8 | fn main(_handle: Handle, _st: SystemTable<Boot>) -> bool {
-  |    ^^^^ expected struct `Status`, found `bool`
+  |    ^^^^ expected fn pointer, found fn item
   |
   = note: expected fn pointer `extern "efiapi" fn(uefi::Handle, uefi::table::SystemTable<_>) -> Status`
                 found fn item `extern "efiapi" fn(uefi::Handle, uefi::table::SystemTable<_>) -> bool {main}`
+  = note: when the arguments and return types match, functions can be coerced to function pointers