fix typo in README.md

Co-authored-by: Kenneth Hoste <[email protected]>

README.md

@@ -656,7 +656,7 @@ initial_comment = New job on instance `{app_name}` for architecture `{arch_name}
 ```
 `initial_comment` is used to create a comment to a PR when a new job has been
 created. Note, the part '{accelerator_spec}' is only filled-in by the bot if the
-argument 'accelerator' to the `bot: build` command have been used.
+argument 'accelerator' to the `bot: build` command has been used.
 ```
 with_accelerator =  and accelerator `{accelerator}`
 ```