Update if.md (#2286)

The original phrasing may imply that you have to always terminated a
`if` expression with `;`. But the real reason that we have to do that is
because we are using it in a `let` statement here, and `let` statement
has to be terminated by `;`

---------

Co-authored-by: Martin Geisler <[email protected]>

Author: JasonXJ

src/control-flow-basics/if.md

@@ -38,8 +38,8 @@ Because `if` is an expression and must have a particular type, both of its
 branch blocks must have the same type. Show what happens if you add `;` after
 `"small"` in the second example.
 
-When `if` is used in an expression, the expression must have a `;` to separate
-it from the next statement. Remove the `;` before `println!` to see the compiler
-error.
+An `if` expression should be used in the same way as the other expressions. For
+example, when it is used in a `let` statement, the statement must be terminated
+with a `;` as well. Remove the `;` before `println!` to see the compiler error.
 
 </details>