address review comment: fix warning message

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__assert__bool_literal.snap

···
 3 │   assert True
   │          ^^^^
 
-Asserting on a literal Boolean is redundant since you can already tell
-whether it will be True or False.
+Asserting on a literal bool is redundant since you can already tell whether
+it will be `True` or `False`.

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__assert__negation_of_bool_literal.snap

···
 3 │   assert !False
   │          ^^^^^^
 
-Asserting on a literal Boolean is redundant since you can already tell
-whether it will be True or False.
+Asserting on a literal bool is redundant since you can already tell whether
+it will be `True` or `False`.

compiler-core/src/warning.rs

···
                 type_::Warning::AssertLiteralBool { location } => Diagnostic {
                     title: "Assertion of a literal value".into(),
                     text: wrap(
-                        "Asserting on a literal Boolean is redundant since you \
-can already tell whether it will be True or False.",
+                        "Asserting on a literal bool is redundant since you \
+can already tell whether it will be `True` or `False`.",
                     ),
                     hint: None,
                     level: diagnostic::Level::Warning,