Fix typo

Rationale is an explantio, rational means "consistent with reason"

Author: waynew

aiosmtpd/docs/NEWS.rst

@@ -14,7 +14,7 @@ Added
 
 Fixed/Improved
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-* All Controllers now have more rationale design, as they are now composited from a Base + a Mixin
+* All Controllers now have more rational design, as they are now composited from a Base + a Mixin
 * A whole bunch of annotations