minor symfony#18169 Fix leftover conflict marker in UPGRADE-3.0.md (guilliamxavier)

This PR was merged into the 3.0 branch.

Discussion
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Fix leftover conflict marker in UPGRADE-3.0.md

| Q             | A
| ------------- | ---
| Branch        | 3.0
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   |
| Fixed tickets |
| License       | MIT
| Doc PR        |

Presumably after resolving a merge conflict, a parasite line `>>>>>>> 2.8` was introduced as part of commit 67df429:
* symfony@67df429#diff-0901ceb76939b17cd920bc69aa52d21fR406
* https://github.com/symfony/symfony/blame/67df429d99c1cf55227f8bb065f88872e5b3edeb/UPGRADE-3.0.md#L406

The present PR just deletes this line 406.

Commits
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b47220a Fix leftover conflict marker in UPGRADE-3.0.md

Author: fabpot

UPGRADE-3.0.md

@@ -403,7 +403,6 @@ UPGRADE FROM 2.x to 3.0
 
  * The `choice_list` option of `ChoiceType` was removed.
 
->>>>>>> 2.8
  * The option "precision" was renamed to "scale".
 
    Before: