Fix typo in "Basic CSS Selectors" description (#26)

# Summary

In the resource description for "Basic CSS Selectors", the word `introducing` is incorrect and should be replaced with `introduction`.

## Problem

The sentence, `An introducing to the very basic CSS selectors you need to know.`, is incorrectly using the word `introducing`.

## Solution

The sentence should read, `An introduction to the very basic CSS selectors you need to know.`

Author: DarrickFauvel

readme.md

@@ -43,7 +43,7 @@ This list is mainly about [CSS](https://developer.mozilla.org/docs/Web/CSS) –
 
 ## Selectors
 
-- [Basic CSS Selectors](https://www.sitepoint.com/css-selectors/) - An introducing to the very basic CSS selectors you need to know.
+- [Basic CSS Selectors](https://www.sitepoint.com/css-selectors/) - An introduction to the very basic CSS selectors you need to know.
 - [Advanced CSS Selectors](https://www.smashingmagazine.com/2009/08/taming-advanced-css-selectors/) - Level up your knowledge. From attribute selectors to CSS3 pseudo classes.
 - [CSS Diner](https://flukeout.github.io) - Learn how to use CSS selectors with this fun little game.