add an example to setControlCHook (#19416)

* add an example to setControlCHook

* [skip CI] format example for setControlCHook

Co-authored-by: Nathan Blaxall <nathan.blaxall@actionstep.com>

lib/system.nim

@@ -2319,6 +2319,15 @@ when notJSnotNims:
   proc setControlCHook*(hook: proc () {.noconv.})
     ## Allows you to override the behaviour of your application when CTRL+C
     ## is pressed. Only one such hook is supported.
+    ## Example:
+    ##
+    ## .. code-block:: Nim
+    ##   proc ctrlc() {.noconv.} =
+    ##     echo "Ctrl+C fired!"
+    ##     # do clean up stuff
+    ##     quit()
+    ##
+    ##   setControlCHook(ctrlc)
 
   when not defined(noSignalHandler) and not defined(useNimRtl):
     proc unsetControlCHook*()