Fix Python install instruction (#1547)

In #1526 we moved the location of the python package, so should also update the install instructions.

- Closes #1546

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - Mark Harris (https://github.com/harrism)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #1547

README.md

@@ -127,7 +127,8 @@ $ make test
 
 - Build, install, and test the `rmm` python package, in the `python` folder:
 ```bash
-$ python -m pip install -e ./python
+# In the root rmm directory
+$ python -m pip install -e ./python/rmm
 $ pytest -v
 ```