Fix some misuses of the indefinite article (#2497)

Fixes two typos.

Author: mateusfccp

specification/dartLangSpec.tex

@@ -5798,7 +5798,7 @@ \section{Extensions}
 e.g., in order to access static members of the extension,
 or in order to resolve an invocation explicitly%
 }).
-A compile-time error occurs if the declared name of a extension
+A compile-time error occurs if the declared name of an extension
 is a built-in identifier.
 
 \LMHash{}%
@@ -6092,7 +6092,7 @@ \subsection{Explicit Invocation of an Instance Member of an Extension}
 }
 
 
-\subsection{Implicit Invocation of an Instance Member of a Extension}
+\subsection{Implicit Invocation of an Instance Member of an Extension}
 \LMLabel{implicitExtensionInvocations}
 
 \LMHash{}%