Merge pull request #20282 from anantoghosh/patch-2

Migration docs: Fix typo in 'Reboot' about units

docs/migration.md

@@ -59,7 +59,7 @@ This list highlights key changes by component between v3.x.x and v4.0.0.
 
 ### Reboot
 
-New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with our own somewhat opinionated reset styles. Selectors appearing in this file only use elements—there are no classes here. This isolates our reset styles from our component styles for a more modular approach. Some of the most important resets this includes are the `box-sizing: border-box` change, moving from `rem` to `em` units on many elements, link styles, and many form element resets.
+New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with our own somewhat opinionated reset styles. Selectors appearing in this file only use elements—there are no classes here. This isolates our reset styles from our component styles for a more modular approach. Some of the most important resets this includes are the `box-sizing: border-box` change, moving from `em` to `rem` units on many elements, link styles, and many form element resets.
 
 ### Typography