Update README.md

Co-authored-by: Copilot <[email protected]>

Author: mdboom

README.md

@@ -137,7 +137,7 @@ For each runner in your `bench_runner.toml`, you can specify a `plot` table with
 
 - `name`: A human-friendly name to display in the plot legend
 - `style`: A [matplotlib line style](https://matplotlib.org/stable/api/_as_gen/matplotlib.lines.Line2D.html#matplotlib.lines.Line2D.set_linestyle)
-- `marker` A [matplotlib marker](https://matplotlib.org/stable/api/markers_api.html#module-matplotlib.markers)
+- `marker`: A [matplotlib marker](https://matplotlib.org/stable/api/markers_api.html#module-matplotlib.markers)
 - `color: A [matplotlib color](https://matplotlib.org/stable/users/explain/colors/colors.html#colors-def)
 
 #### Longitudinal plot configuration