rfc0010: fix typo

Signed-off-by: Filipe Laíns <lains@riseup.net>

rfcs/0010-adopt-pep517-tooling.rst

@@ -26,7 +26,7 @@ remove support for them in the future. Please see `setuptools #2088`_ and
 
 The metadata installed by ``setup.py install`` is the old-style "egg" format
 (.egg-info). Is is essentially implementation-defined by setuptools and has been
-replaced a standardized format (.dist-info).
+replaced by a standardized format (.dist-info).
 By moving to the PEP 517 tooling, we will be using the wheel format, which has
 the standardized metadata.