Add 2021 package to go.md (Azure#12830)

* Add 2021 package to go.md

* fix missed package

* change output folder

* fix typo

specification/mixedreality/resource-manager/readme.go.md

@@ -13,10 +13,20 @@ go:
 
 ``` yaml $(go) && $(multiapi)
 batch:
+  - tag: package-2021-01
   - tag: package-2020-05
   - tag: package-2019-02-preview
 ```
 
+### Tag: package-2021-01 and go
+
+These settings apply only when `--tag=package-2021-01 --go` is specified on the command line.
+Please also specify `--go-sdk-folder=<path to the root directory of your azure-sdk-for-go clone>`.
+
+``` yaml $(tag) == 'package-2021-01' && $(go)
+output-folder: $(go-sdk-folder)/services/$(namespace)/mgmt/2021-01-01/$(namespace)
+```
+
 ### Tag: package-2020-05 and go
 
 These settings apply only when `--tag=package-2020-05 --go` is specified on the command line.