Use code font for jl_value_t in h2 (#28702)

By just using text, the underscores around value are interpreted as begin/end italics.

(cherry picked from commit 1fee8a4)

Author: dhermes

doc/src/devdocs/object.md

@@ -1,6 +1,6 @@
 # Memory layout of Julia Objects
 
-## Object layout (jl_value_t)
+## Object layout (`jl_value_t`)
 
 The `jl_value_t` struct is the name for a block of memory owned by the Julia Garbage Collector,
 representing the data associated with a Julia object in memory. Absent any type information, it