Clean up some uses on 'an' in builder docs
@@ -239,13 +239,13 @@ In the case where `<src>` is a remote file URL, the destination will have permis
> **Note**:
> If you build using STDIN (`docker build - < somefile`), there is no
-> build context, so the Dockerfile can only contain an URL based ADD
+> build context, so the Dockerfile can only contain a URL based ADD
> statement.
> **Note**:
> If your URL files are protected using authentication, you will need to
-> use an `RUN wget` , `RUN curl`
-> or other tool from within the container as ADD does not support
+> use `RUN wget` , `RUN curl`
+> or use another tool from within the container as ADD does not support
> authentication.
The copy obeys the following rules:
