Inline Code in Docstrings

[seanlaw]

Currently, the docstring found in the STUMPY API page uses single backticks to denote inline code (e.g., `m`). However, this is incorrect and should be changed to double backticks so that things like variables can be rendered as inline code (e.g., ``m``).

Note that this should be done AFTER #986 is merged for fixing #772!