Update api.md

Author: ytdHuang

docs/src/resources/api.md

@@ -314,6 +314,7 @@ AbstractLinearMap
 QuantumToolbox.settings
 QuantumToolbox.versioninfo
 QuantumToolbox.about
+QuantumToolbox.cite
 gaussian
 n_thermal
 PhysicalConstants
@@ -342,4 +343,4 @@ add_line!
 add_arc!
 add_states!
 clear!
-```
+```