Fix typo

Author: ma-sadeghi

README.md

@@ -61,10 +61,13 @@ Lists the slowest tests based on the information stored in the test durations fi
 ## Interactions with other pytest plugins
 * [`pytest-random-order`](https://github.com/jbasko/pytest-random-order): ⚠️ The **default settings** of that plugin (setting only `--random-order` to activate it) are **incompatible** with `pytest-split`. Test selection in the groups happens after randomization, potentially causing some tests to be selected in several groups and others not at all. Instead, a global random seed needs to be computed before running the tests (for example using `$RANDOM` from the shell) and that single seed then needs to be used for all groups by setting the `--random-order-seed` option.
 
-* [`nbval`](https://github.com/computationalmodelling/nbval): `pytest-split` could, in principle, break up a single IPython Notebook into different test groups. This most likely causes broken up pieces to fail (for the very least, package `import`s are usually done at Cell 0, and so, any broken up piece that doesn't contain Cell 0 will certainly fail).). To avoid this, after splitting step is done, test groups are reorganized based on a simple algorithm illustrated in the following cartoon:
+* [`nbval`](https://github.com/computationalmodelling/nbval): `pytest-split` could, in principle, break up a single IPython Notebook into different test groups. This most likely causes broken up pieces to fail (for the very least, package `import`s are usually done at Cell 1, and so, any broken up piece that doesn't contain Cell 1 will certainly fail). To avoid this, after splitting step is done, test groups are reorganized based on a simple algorithm illustrated in the following cartoon:
 
 ![image](https://user-images.githubusercontent.com/14086031/145830494-07afcaf0-5a0f-4817-b9ee-f84a459652a8.png)
 
+where letters A to E refer to an individual IPython Notebook, and the numbers refer
+to the corresponding cell number.
+
 ## Splitting algorithms
 The plugin supports multiple algorithms to split tests into groups.
 Each algorithm makes different tradeoffs, but generally `least_duration` should give more balanced groups.