ch7: fix code format

ch7/ch7-13.md

@@ -12,10 +12,10 @@
 import "database/sql"
 
 func listTracks(db sql.DB, artist string, minYear, maxYear int) {
-    result, err := db.Exec(
-        "SELECT * FROM tracks WHERE artist = ? AND ? <= year AND year <= ?",
-        artist, minYear, maxYear)
-    // ...
+	result, err := db.Exec(
+		"SELECT * FROM tracks WHERE artist = ? AND ? <= year AND year <= ?",
+		artist, minYear, maxYear)
+	// ...
 }
 ```
 
@@ -23,22 +23,22 @@ Exec方法使用SQL字面量替換在査詢字符串中的每個'?'；SQL字面
 
 ```go
 func sqlQuote(x interface{}) string {
-    if x == nil {
-        return "NULL"
-    } else if _, ok := x.(int); ok {
-        return fmt.Sprintf("%d", x)
-    } else if _, ok := x.(uint); ok {
-        return fmt.Sprintf("%d", x)
-    } else if b, ok := x.(bool); ok {
-        if b {
-            return "TRUE"
-        }
-        return "FALSE"
-    } else if s, ok := x.(string); ok {
-        return sqlQuoteString(s) // (not shown)
-    } else {
-        panic(fmt.Sprintf("unexpected type %T: %v", x, x))
-    }
+	if x == nil {
+		return "NULL"
+	} else if _, ok := x.(int); ok {
+		return fmt.Sprintf("%d", x)
+	} else if _, ok := x.(uint); ok {
+		return fmt.Sprintf("%d", x)
+	} else if b, ok := x.(bool); ok {
+		if b {
+			return "TRUE"
+		}
+		return "FALSE"
+	} else if s, ok := x.(string); ok {
+		return sqlQuoteString(s) // (not shown)
+	} else {
+		panic(fmt.Sprintf("unexpected type %T: %v", x, x))
+	}
 }
 ```
 
@@ -48,11 +48,11 @@ switch語句可以簡化if-else鏈，如果這個if-else鏈對一連串值做相
 
 ```go
 switch x.(type) {
-    case nil:       // ...
-    case int, uint: // ...
-    case bool:      // ...
-    case string:    // ...
-    default:        // ...
+	case nil:       // ...
+	case int, uint: // ...
+	case bool:      // ...
+	case string:    // ...
+	default:        // ...
 }
 ```
 
@@ -70,21 +70,21 @@ switch x := x.(type) { /* ... */ }
 
 ```go
 func sqlQuote(x interface{}) string {
-    switch x := x.(type) {
-    case nil:
-        return "NULL"
-    case int, uint:
-        return fmt.Sprintf("%d", x) // x has type interface{} here.
-    case bool:
-        if x {
-            return "TRUE"
-        }
-        return "FALSE"
-    case string:
-        return sqlQuoteString(x) // (not shown)
-    default:
-        panic(fmt.Sprintf("unexpected type %T: %v", x, x))
-    }
+	switch x := x.(type) {
+	case nil:
+		return "NULL"
+	case int, uint:
+		return fmt.Sprintf("%d", x) // x has type interface{} here.
+	case bool:
+		if x {
+			return "TRUE"
+		}
+		return "FALSE"
+	case string:
+		return sqlQuoteString(x) // (not shown)
+	default:
+		panic(fmt.Sprintf("unexpected type %T: %v", x, x))
+	}
 }
 ```