docs: Fix typo in book (#1537)

* docs: Fix typo in book

* Update book/src/usage.md

Co-authored-by: Eric Crosson <EricCrosson@users.noreply.github.com>

Co-authored-by: Blaž Hrastnik <blaz@mxxn.io>
Co-authored-by: Eric Crosson <EricCrosson@users.noreply.github.com>

book/src/usage.md

@@ -42,7 +42,7 @@ helix. The keymappings have been inspired from [vim-sandwich](https://github.com
 `ms` acts on a selection, so select the text first and use `ms<char>`. `mr` and `md` work
 on the closest pairs found and selections are not required; use counts to act in outer pairs.
 
-It can also act on multiple seletions (yay!). For example, to change every occurance of `(use)` to `[use]`:
+It can also act on multiple selections (yay!). For example, to change every occurrence of `(use)` to `[use]`:
 
 - `%` to select the whole file
 - `s` to split the selections on a search term