Remove axios.all() and axios.spread() from Readme.md (#2727)

* Updating Readme.md
- remove axios.all(), axios.spread()

* Updating Readme.md
- replace example
- axios.all() -> Promise.all()
- axios.spread(function (acct, perms)) -> function (acct, perms)
- add deprecated mark

* Update README.md

Make changes after review

Co-authored-by: Jay <jasonsaayman@gmail.com>

README.md

@@ -148,10 +148,11 @@ function getUserPermissions() {
   return axios.get('/user/12345/permissions');
 }
 
-axios.all([getUserAccount(), getUserPermissions()])
-  .then(axios.spread(function (acct, perms) {
-    // Both requests are now complete
-  }));
+Promise.all([getUserAccount(), getUserPermissions()])
+  .then(function (results) {
+    const acct = results[0];
+    const perm = results[1];
+  });
 ```
 
 ## axios API
@@ -207,12 +208,13 @@ For convenience aliases have been provided for all supported request methods.
 ###### NOTE
 When using the alias methods `url`, `method`, and `data` properties don't need to be specified in config.
 
-### Concurrency
+### Concurrency (Deprecated)
+Please use `Promise.all` to replace the below functions.
 
 Helper functions for dealing with concurrent requests.
 
-##### axios.all(iterable)
-##### axios.spread(callback)
+axios.all(iterable)
+axios.spread(callback)
 
 ### Creating an instance