docs: trimmed some white spaces (#4070)

docs/doc.md

@@ -172,7 +172,7 @@ func main() {
   router := gin.Default()
 
   // Query string parameters are parsed using the existing underlying request object.
-  // The request responds to an url matching:  /welcome?firstname=Jane&lastname=Doe
+  // The request responds to an url matching: /welcome?firstname=Jane&lastname=Doe
   router.GET("/welcome", func(c *gin.Context) {
     firstname := c.DefaultQuery("firstname", "Guest")
     lastname := c.Query("lastname") // shortcut for c.Request.URL.Query().Get("lastname")
@@ -615,7 +615,7 @@ You can also specify that specific fields are required. If a field is decorated
 ```go
 // Binding from JSON
 type Login struct {
-  User     string `form:"user" json:"user" xml:"user"  binding:"required"`
+  User     string `form:"user" json:"user" xml:"user" binding:"required"`
   Password string `form:"password" json:"password" xml:"password" binding:"required"`
 }
 
@@ -1252,7 +1252,7 @@ func main() {
 
 #### JSONP
 
-Using JSONP to request data from a server  in a different domain. Add callback to response body if the query parameter callback exists.
+Using JSONP to request data from a server in a different domain. Add callback to response body if the query parameter callback exists.
 
 ```go
 func main() {
@@ -1301,7 +1301,7 @@ func main() {
 
 #### PureJSON
 
-Normally, JSON replaces special HTML characters with their unicode entities, e.g. `<` becomes  `\u003c`. If you want to encode such characters literally, you can use PureJSON instead.
+Normally, JSON replaces special HTML characters with their unicode entities, e.g. `<` becomes `\u003c`. If you want to encode such characters literally, you can use PureJSON instead.
 This feature is unavailable in Go 1.6 and lower.
 
 ```go