Fix example code on exceptions

`Can not resolve type Exception` was given, related issue #87.

Author: jasalt

content/documentation/control-flow.md

@@ -172,16 +172,16 @@ All expressions are evaluated and if no exception is thrown the value of the las
 (try) # evaluates to nil
 
 (try
-  (throw (php/new Exception))
-  (catch Exception e "error")) # evaluates to "error"
+  (throw (php/new \Exception))
+  (catch \Exception e "error")) # evaluates to "error"
 
 (try
   (+ 1 1)
   (finally (print "test"))) # Evaluates to 2 and prints "test"
 
 (try
-  (throw (php/new Exception))
-  (catch Exception e "error")
+  (throw (php/new \Exception))
+  (catch \Exception e "error")
   (finally (print "test"))) # evaluates to "error" and prints "test"
 ```