fix some bugs

ch04/README.md

@@ -19,7 +19,7 @@ and potential pitfalls in the use of the language by the programmer.
 Do you consider that an acceptable trade-off? Why or why not?
 
 Yes, I think it necessary to hold the trade-off.
-Because the speed always the biggest advantage of C++. Sometimes, we need
+Because the speed is always the biggest advantage of C++. Sometimes, we need
 the compiler's features for efficient work. But if you are not a expert. I
 have to advice you do not touch the undefined behaviors.
 
@@ -130,7 +130,7 @@ dval = ival = pi = 0;
 // can not assign to 'int' from type 'int *'
 // correct it:
 dval = ival = 0;
-pi = &ival;
+pi = 0;
 ```
 
 ##Exercise 4.16
@@ -264,7 +264,7 @@ reference: [Why the size of a pointer is 4bytes in C++](http://stackoverflow.com
 sizeof x + y      // sizeof(x+y)
 sizeof p->mem[i]  // sizeof(p->mem[i])
 sizeof a < b      // sizeof(a) < b
-sizeof f()        // compile error
+sizeof f()        // compile successfully.
 ```
 
 -----