Fixed a typo and corrected method name in step 6 (SharePoint#2083)

Fixed mis-spelled word bild to build
Step 6 says that the variables can be scoped within onRender method. But in SPFx extensions we do not recommend using OnRender method and especially in this particular sample it supposed to _renderPlaceHolders method.

Author: svarukala

docs/spfx/extensions/get-started/using-page-placeholder-with-extensions.md

@@ -135,7 +135,6 @@ Notice that you're requesting a well-known placeholder by using the correspondin
       private _bottomPlaceholder: PlaceholderContent | undefined;
   ```
 
-
 8. Update the `onInit` method code as follows:
 
   ```typescript
@@ -238,7 +237,7 @@ Notice that you're requesting a well-known placeholder by using the correspondin
 	* Extension code should not assume that the expected placeholder is available.
 	* The code expects custom properties called `Top` and `Bottom`. If the properties exist, they render inside the placeholders.
 	* Notice that the code path for both the top and bottom placeholders is almost identical. The only differences are the variables used and the style definitions.
-	* It is possible to use the class names defined in the style sheet directly but it is not recommended. In case no style sheet reference defined in the ```styles``` variable is found in the code, the style sheet won't get added to the page. This is because unused references will get removed during bild process.
+	* It is possible to use the class names defined in the style sheet directly but it is not recommended. In case no style sheet reference defined in the ```styles``` variable is found in the code, the style sheet won't get added to the page. This is because unused references will get removed during build process.
 
 10. Add the following method after the `_renderPlaceHolders` method. In this case, you simply output a console message when the extension is removed from the page.