01-margin-and-padding-1: Fix typo in the README

Rename "Self-check" to "Self check" to keep it in line with the other files.

Author: gs-101

foundations/block-and-inline/01-margin-and-padding-1/README.md

@@ -4,7 +4,7 @@ For this first exercise, simply edit the `style.css` file so that the divs look
 
 ![outcome](./desired-outcome.png)
 
-### Self-check 
+### Self check
 Use this section to check your work. On _these_ projects, your goal isn't to attain 100% pixel perfection, but to use the tools you've learned to get relatively close to the desired output.
 
 - [ ] Div One and Div Three have 32px between their text and border.