minor #9312 [Form] Fix reStructuredText markup (julienfalque)

This PR was submitted for the 3.3 branch but it was merged into the 3.4 branch instead (closes #9312).

Discussion
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[Form] Fix reStructuredText markup

Commits
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97ab525 Fix reStructuredText markup

Author: javiereguiluz

form/unit_testing.rst

@@ -206,7 +206,7 @@ allows you to return a list of extensions to register::
 
 It is also possible to load custom form types, form type extensions or type
 guessers using the :method:`Symfony\\Component\\Form\\Test\\FormIntegrationTestCase::getTypes`,
-`:method:`Symfony\\Component\\Form\\Test\\FormIntegrationTestCase::`getTypeExtensions`
+:method:`Symfony\\Component\\Form\\Test\\FormIntegrationTestCase::getTypeExtensions`
 and :method:`Symfony\\Component\\Form\\Test\\FormIntegrationTestCase::getTypeGuessers`
 methods.