Apply suggestions from code review

Author: tamird

library/core/src/fmt/rt.rs

@@ -81,9 +81,9 @@ macro_rules! argument_new {
                 // cost of this helper function (mainly code-size) when it is not needed, we
                 // transmute the function pointer otherwise.
                 //
-                // This is is similar to what the Rust compiler does internally with vtables when
-                // CFI is enabled, where it generates trampoline functions that only serve to adjust
-                // the expected type of the argument. `ArgumentType::Placeholder` is a bit like a
+                // This is similar to what the Rust compiler does internally with vtables when CFI
+                // is enabled, where it generates trampoline functions that only serve to adjust the
+                // expected type of the argument. `ArgumentType::Placeholder` is a bit like a
                 // manually constructed trait object, so it is not surprising that the same approach
                 // has to be applied here as well.
                 //