Simplify TestPyPI install instructions to just extra-index-url (#1944)

The default `--index-url` is https://pypi.org/simple, so just need to set `--extra-index-url` to point to TestPyPI.

Author: weiji14

doc/install.rst

@@ -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.