remove useless alloc

on rust reference book this reduces total allocs from 490mb to 474mb:

==23272== Total:     490,538,699 bytes in 1,760,117 blocks
==23272== At t-gmax: 13,872,954 bytes in 4,655 blocks
==23272== At t-end:  488,516 bytes in 884 blocks
==23272== Reads:     830,509,060 bytes
==23272== Writes:    522,290,614 bytes

to

==40876== Total:     474,156,323 bytes in 1,521,025 blocks
==40876== At t-gmax: 13,872,954 bytes in 4,655 blocks
==40876== At t-end:  488,516 bytes in 884 blocks
==40876== Reads:     820,933,434 bytes
==40876== Writes:    514,838,350 bytes

src/renderer/html_handlebars/helpers/navigation.rs

@@ -103,7 +103,7 @@ fn find_chapter(
                     }
                 }
 
-                previous = Some(item.clone());
+                previous = Some(item);
             }
             _ => continue,
         }