Apply suggestions from code review

Co-authored-by: Nikola Matić <nikola.matic@ethereum.org>

Author: matheusaaguiar

docs/contracts/custom-storage-layout.rst

@@ -20,7 +20,7 @@ will be stored from the specified slot instead of the default slot zero.
 As the previous example shows, this can be done by using ``layout at <base-slot-expression>``
 in the header of a contract definition.
 
-The layout specifier can given at most once and can be either after or before the inheritance specifier.
+The layout specifier can be placed either before or after the inheritance specifier, and at most once.
 The ``base-slot-expression`` must be an :ref:`integer literal<rational_literals>` expression
 that can be evaluated at compile time and yield a value in the range of ``uint256``.