perl: Fix typo in Carton example

Ref Perl/docker-perl#98

perl/content.md

@@ -77,7 +77,7 @@ RUN cpanm Carton \
     && mkdir -p /usr/src/app
 WORKDIR /usr/src/app
 
-ONBUILD COPY cpanfile* /usr/src/myapp
+ONBUILD COPY cpanfile* /usr/src/app
 ONBUILD RUN carton install
 
 ONBUILD COPY . /usr/src/app