minor symfony#15497 typofix - https://github.com/vlajos/misspell_fixer (vlajos)

This PR was submitted for the 2.8 branch but it was merged into the 2.3 branch instead (closes symfony#15497).

Discussion
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typofix - https://github.com/vlajos/misspell_fixer

Fix 3 small typos. Original:  symfony#15494

Commits
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0692ca9 typofix - https://github.com/vlajos/misspell_fixer

Author: nicolas-grekas

src/Symfony/Component/Validator/Tests/Mapping/ClassMetadataTest.php

@@ -113,7 +113,7 @@ public function testMemberMetadatas()
         $this->metadata->addPropertyConstraint('firstName', new ConstraintA());
 
         $this->assertTrue($this->metadata->hasMemberMetadatas('firstName'));
-        $this->assertFalse($this->metadata->hasMemberMetadatas('non_existant_field'));
+        $this->assertFalse($this->metadata->hasMemberMetadatas('non_existent_field'));
     }
 
     public function testMergeConstraintsKeepsPrivateMembersSeparate()

src/Symfony/Component/Yaml/Tests/Fixtures/YtsDocumentSeparator.yml

@@ -21,7 +21,7 @@ ruby: |
 test: Leading Document Separator
 todo: true
 brief: >
-    You can explicity give an opening
+    You can explicitly give an opening
     document separator to your YAML stream.
 yaml: |
     ---