Surround a function call with backticks

Co-authored-by: Kirill Mikhailov <[email protected]>

esp-alloc/src/lib.rs

@@ -561,7 +561,7 @@ unsafe impl GlobalAlloc for EspHeap {
             {
                 let mut internal_heap_stats = self.internal_heap_stats.borrow_ref_mut(cs);
                 drop(regions);
-                // We need to call used because [linked_list_allocator::Heap] does internal size
+                // We need to call `used()` because [linked_list_allocator::Heap] does internal size
                 // alignment so we cannot use the size provided by the layout.
                 internal_heap_stats.total_freed += before - self.used();
             }