README.rst: pip install --upgrade

[cclauss]

-   pip install -U drf-yasg
+   pip install --upgrade drf-yasg

pip install currently has six --u* options so in code that will be read by others, let's use the most self-documenting form of the command.

% pip install --help | grep "\-\-u"

                              <dir>. Use --upgrade to replace existing
  --user                      Install to the Python user install directory for
  -U, --upgrade               Upgrade all specified packages to the newest
  --upgrade-strategy <upgrade_strategy>
  --use-pep517                Use PEP 517 for building source distributions
  --use-feature <feature>     Enable new functionality, that may be backward
  --use-deprecated <feature>  Enable deprecated functionality, that will be

[JoelLefkowitz]

@cclauss Thank you