Fix run-on sentences (#7792)

Author: gitname

docs/src/pages/customization/css-in-js.md

@@ -14,7 +14,7 @@ We think that it's the future:
 - [The future of component-based styling](https://medium.freecodecamp.com/css-in-javascript-the-future-of-component-based-styling-70b161a79a32)
 
 So, you may have noticed in the demos how this *CSS-in-JS* looks.
-We use the `withStyles` Higher-order Component, here is an example:
+We use the `withStyles` Higher-order Component. Here's an example:
 
 {{demo='pages/customization/CssInJs.js'}}
 
@@ -45,7 +45,7 @@ When only rendering on the client, that's not something you need to be aware of.
 ## Class names
 
 You may have noticed, the class names generated by our styling solution are **non-deterministic**.
-You can't rely on their name, the following CSS won't work.
+You can't rely on them. The following CSS won't work:
 ```css
 .MuiAppBar-root-12 {
   opacity: 0.6