Update a3-reference.md (#6309)

Co-authored-by: Yong He <yonghe@outlook.com>

Author: KoT3isGood

docs/user-guide/a3-reference.md

@@ -7,5 +7,5 @@ Reference
 ============================
 
 In this chapter:
-1. [Reference for all Capability Profiles](a3-01-Reference-Capability-Profiles)
-2. [Reference for all Capability Atoms](a3-02-Reference-Capability-Atoms)
+1. [Reference for all Capability Profiles](a3-01-Reference-Capability-Profiles.md)
+2. [Reference for all Capability Atoms](a3-02-Reference-Capability-Atoms.md)