Fixed grammar.

Author: simphotonics

README.md

@@ -61,7 +61,7 @@ process.
 The extension [`Reader`][Reader] provides a systematic method of
 retrieving constants of
 arbitrary data-types by allowing users to register [`Decoder`][Decoder] objects.
-To create custom decoder extend [`Decoder<T>`][Decoder] and override the
+To create a custom decoder extend [`Decoder<T>`][Decoder] and override the
 the method `T read<T>(DartObject obj)`. This method
 attempts to read a variable of type `T` from the compile-time constant
 `obj` and returns the result.