Fix some nit comments and format errors to split the static assert across multiple lines

Author: Tomek-Stolarczyk

include/fmt/format.h

@@ -553,7 +553,8 @@ FMT_CONSTEXPR auto fill_n(OutputIt out, Size count, const T& value)
 template <typename T, typename Size>
 FMT_CONSTEXPR20 auto fill_n(T* out, Size count, char value) -> T* {
   if (is_constant_evaluated()) return fill_n<T*, Size, T>(out, count, value);
-  static_assert(1 ==  sizeof(T), "sizeof(T) must be 1 to use char for initialization");
+  static_assert(sizeof(T) == 1,
+                "sizeof(T) must be 1 to use char for initialization");
   std::memset(out, value, to_unsigned(count));
   return out + count;
 }