minor #31339 Reword VarDumper description (greg0ire)

This PR was merged into the 3.4 branch.

Discussion
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Reword VarDumper description

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

It is hard to understand what "Built on top" refers to, and even when
knowing, the sentence looks weird.
![Untitled](https://user-images.githubusercontent.com/657779/57011659-15ebfe00-6c03-11e9-9b85-5c00cfc15026.png)

The description of [the Github repository](https://github.com/symfony/var-dumper) has the same issue and should be changed.

Commits
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6024e16 Reword VarDumper description

src/Symfony/Component/VarDumper/README.md

@@ -2,8 +2,8 @@ VarDumper Component
 ===================
 
 The VarDumper component provides mechanisms for walking through any arbitrary
-PHP variable. Built on top, it provides a better `dump()` function that you
-can use instead of `var_dump`.
+PHP variable. It provides a better `dump()` function that you can use instead
+of `var_dump`.
 
 Resources
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