add a code quote to a word (#28699)

Author: KristofferC

base/essentials.jl

@@ -795,7 +795,7 @@ isdone(itr, state...) = missing
     iterate(iter [, state]) -> Union{Nothing, Tuple{Any, Any}}
 
 Advance the iterator to obtain the next element. If no elements
-remain, nothing should be returned. Otherwise, a 2-tuple of the
+remain, `nothing` should be returned. Otherwise, a 2-tuple of the
 next element and the new iteration state should be returned.
 """
 function iterate end