minor #6281 Change isValid to isSubmitted. (mustafaaloko)

This PR was submitted for the 2.8 branch but it was merged into the 2.3 branch instead (closes #6281).

Discussion
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Change isValid to isSubmitted.

Fixed this line: `isValid()` returns false if the form was not submitted.
To this: `isSubmitted()` returns false if the form was not submitted.

Commits
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f44ec98 Change isValid to isSubmitted.

book/forms.rst

@@ -260,7 +260,7 @@ possible paths:
 #. When initially loading the page in a browser, the form is simply created and
    rendered. :method:`Symfony\\Component\\Form\\FormInterface::handleRequest`
    recognizes that the form was not submitted and does nothing.
-   :method:`Symfony\\Component\\Form\\FormInterface::isValid` returns ``false``
+   :method:`Symfony\\Component\\Form\\FormInterface::isSubmitted` returns ``false``
    if the form was not submitted.
 
 #. When the user submits the form, :method:`Symfony\\Component\\Form\\FormInterface::handleRequest`