fix: avoid unused variable warning

en/src/basic-types/functions.md

@@ -85,7 +85,7 @@ fn main() {
     // FILL in the blank
     let b = __;
 
-    let v = match b {
+    let _v = match b {
         true => 1,
         // Diverging functions can also be used in match expression to replace a value of any value
         false => {

solutions/basic-types/functions.md

@@ -108,7 +108,7 @@ fn main() {
     // FILL in the blank
     let b = false;
 
-    let v = match b {
+    let _v = match b {
         true => 1,
         // Diverging functions can also be used in match expression
         false => {

zh-CN/src/basic-types/functions.md

@@ -77,7 +77,7 @@ fn main() {
     // 填空
     let b = __;
 
-    let v = match b {
+    let _v = match b {
         true => 1,
         // 发散函数也可以用于 `match` 表达式，用于替代任何类型的值
         false => {