Fix iterator and container property syntax throughout handbook

syntax/02-datatypes.md

@@ -122,8 +122,8 @@ list.push_back(2)
 list.push_back(3)
 
 # 访问元素
-var first = list.front()
-var last = list.back()
+var first = list.front
+var last = list.back
 
 # 删除元素
 list.pop_back()

syntax/04-control-flow.md

@@ -328,7 +328,7 @@ end
 
 ```covscript
 function findMax(arr)
-    if arr.empty()
+    if arr.empty
         return null
     end
 

syntax/08-memory.md

@@ -437,7 +437,7 @@ class ObjectPool
 
     function acquire()
         if this.available.size > 0
-            var obj = this.available.back()
+            var obj = this.available.back
             this.available.pop_back()
             this.inUse.push_back(obj)
             return obj

syntax/09-exceptions.md

@@ -441,7 +441,7 @@ class HttpClient
     function get(url)
         try
             # 验证 URL
-            if url == null || url.empty()
+            if url == null || url.empty
                 throw new InvalidArgumentException("url", "URL cannot be empty")
             end
 
@@ -485,7 +485,7 @@ end
 ```covscript
 class Validator
     function validateEmail(email)
-        if email == null || email.empty()
+        if email == null || email.empty
             throw new InvalidArgumentException("email", "cannot be empty")
         end