fix syntax error preventing publishing to PyPi

[sarahcollyer]

Proposed Changes

• Fixes a syntax error in the README.rst file that is preventing us publishing the package on PyPi

Related Issues

• Related to #
• Fixes #

Pre-requisites

This section may not be fully required if the branch is not merging into main.
Please indicate items that aren't necessary and why, with comments around incomplete checks.

• Version number has been incremented, according to SemVer
• Changelog has been updated, listing changes to this version. Use the keep a changelog format
• New features are tested
• New features follow the Analysis Function Releasing statistics in spreadsheets guidance
• New features are documented using the numpydoc docstring format
• Other relevant package documentation is updated
• For new functionality, examples are included in the docs or a feature request has
  been made for it/them
• Required workflows and pre-commits succeed