minor #8130 [Form] Fix maxlength attribute (michaelperrin)

This PR was merged into the 2.7 branch.

Discussion
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[Form] Fix maxlength attribute

Surprisingly, some reminiscence of the very old Symfony `max_length` form option (deprecated since Symfony 2.5!) is still present on the forms documentation page (including for Symfony 3.3), despite #3461 was merged and was already fixing it.

These was the only reminiscences, all other pages are up-to-date.

Commits
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f02815e Fix maxlength attribute for forms

Author: xabbuh

forms.rst

@@ -568,10 +568,10 @@ the correct values of a number of field options.
     field ``nullable``). This is very useful, as your client-side validation will
     automatically match your validation rules.
 
-``max_length``
-    If the field is some sort of text field, then the ``max_length`` option can be
-    guessed from the validation constraints (if ``Length`` or ``Range`` is used) or
-    from the Doctrine metadata (via the field's length).
+``maxlength``
+    If the field is some sort of text field, then the ``maxlength`` option attribute
+    can be guessed from the validation constraints (if ``Length`` or ``Range`` is used)
+    or from the Doctrine metadata (via the field's length).
 
 .. caution::