Simplify TestPyPI install instructions to just extra-index-url
The default `--index-url` is so just need to set `--extra-index-url` to point to TestPyPI.
@@ -137,7 +137,7 @@ This installs the latest stable release from
Alternatively, you can install the latest development version from
`TestPyPI <https://test.pypi.org/project/pygmt>`__::
- pip install --pre --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple pygmt
+ pip install --pre --extra-index-url https://test.pypi.org/simple/ pygmt
To upgrade the installed stable release or development version to be the latest
one, just add ``--upgrade`` to the corresponding command above.
