Fix cudf release version in readme (#8331)

As part of this commit 8406522 we accidentally changed the release version of readme to `21.06`, whereas the stable version currently in `rapidsai` channel is `0.19`.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Keith Kraus (https://github.com/kkraus14)

URL: #8331

README.md

@@ -65,15 +65,15 @@ Please see the [Demo Docker Repository](https://hub.docker.com/r/rapidsai/rapids
 
 cuDF can be installed with conda ([miniconda](https://conda.io/miniconda.html), or the full [Anaconda distribution](https://www.anaconda.com/download)) from the `rapidsai` channel:
 
-For `cudf version == 21.06` :
+For `cudf version == 0.19.2` :
 ```bash
-# for CUDA 11.0
+# for CUDA 10.1
 conda install -c rapidsai -c nvidia -c numba -c conda-forge \
-    cudf=21.06 python=3.7 cudatoolkit=11.0
+    cudf=0.19 python=3.7 cudatoolkit=10.1
 
-# or, for CUDA 11.2
+# or, for CUDA 10.2
 conda install -c rapidsai -c nvidia -c numba -c conda-forge \
-    cudf=21.06 python=3.7 cudatoolkit=11.2
+    cudf=0.19 python=3.7 cudatoolkit=10.2
 
 ```