updated build instructions (rapidsai#4200)

Fixes the old build instructions for `cuml`

cc @dantegd

Authors:
  - https://github.com/shaneding

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: rapidsai#4200

BUILD.md

@@ -19,7 +19,8 @@ It is recommended to use conda for environment/package management. If doing so,
 
 ```bash
 conda create -n cuml_dev python=3.7
-conda env update -n cuml_dev --file=conda/environments/cuml_dev_cuda10.2.yml
+conda activate cuml_dev
+conda env update --file=conda/environments/cuml_dev_cuda11.2.yml
 ```
 
 These conda environments are based on the general RAPIDS meta packages that install common dependencies for RAPIDS projects. To install different versions of packages contained in those meta packages after creating the environment, it is recommended to remove those meta packages (without removing the actual packages contained in the environment) with the following command (having the environment active):