Update how-to-add-a-new-rule.md

docs/how-to-add-a-new-rule.md

@@ -19,8 +19,7 @@ package os
 import "github.com/alibaba/opentelemetry-go-auto-instrumentation/api"
 
 func init() {
-	api.NewRule("os", "Setenv", "", "onEnterSetenv", "").
-		Register()
+    api.NewRule("os", "Setenv", "", "onEnterSetenv", "").Register()
 }
 ```
 In the `rule.go` file, we define a new rule for the `os` package. The `NewRule` function takes five arguments: the import path, the function name, the receiver type string, the onEnter function name, and the onExit function name.
@@ -38,12 +37,12 @@ In the `rule.go` file, we define a new rule for the `os` package. The `NewRule`
 package os
 
 import (
-	"fmt"
-	"os"
+    "fmt"
+    "os"
 )
 
 func onEnterSetenv(call os.CallContext, key, value string) {
-	fmt.Printf("Setting environment variable %s to %s", key, value)
+    fmt.Printf("Setting environment variable %s to %s", key, value)
 }
 ```
 
@@ -63,7 +62,7 @@ package main
 
 import (
     ...
-	_ "github.com/alibaba/opentelemetry-go-auto-instrumentation/pkg/rules/os"
+    _ "github.com/alibaba/opentelemetry-go-auto-instrumentation/pkg/rules/os"
 )
 ```
 Dont forget recomplie the tool after adding the new rule, because currently `rules` is coded within the tool.
@@ -85,4 +84,4 @@ Setting environment variable hello to world%
 ```
 The output shows that the rule is working as expected. The `Setting environment variable hello to world` message is printed when the `os.Setenv()` function is called, that is, the probe code is injected successfully.
 
-There are many advanced features that is not covered in this example, such as the `InstStructRule`, `InstFileRule` and APIs of `CallContext`. You can refer to the existing rules in the `pkg/rules` directory for more information.
+There are many advanced features that is not covered in this example, such as the `InstStructRule`, `InstFileRule` and APIs of `CallContext`. You can refer to the existing rules in the `pkg/rules` directory for more information.