Fix markdown in source_maps README.md so it renders correctly in pub site.

pkg/source_maps/README.md

 Source Maps
 ===========
 
 This project implements a Dart pub package to work with source maps. The
 implementation is based on the [source map version 3 spec][spec] which was
 originated from the [Closure Compiler][closure] and has been implemented in
 Chrome and Firefox.
 
 In this package we provide:
+
   * Data types defining file locations and spans: these are not part of the
     original source map specification. These data types are great for tracking
     source locations on source maps, but they can also be used by tools to
     reporting useful error messages that include on source locations.
+

[nweiz, 2014/06/27 18:14:00]

I believe the only change you need to get these list items to be recognized is
to separate them from the previous paragraph, not from one another. See
https://github.com/dart-lang/py-gfm/issues/13

[Siggi Cherem (dart-lang), 2014/06/27 19:07:06]

Done.

   * A builder that creates a source map programatically and produces the encoded
     source map format.
+
   * A parser that reads the source map format and provides APIs to read the
     mapping information.
 
 Some upcoming features we are planning to add to this package are:
+
   * A printer that lets you generate code, but record source map information in
     the process.
+
   * A tool that can compose source maps together. This would be useful for
     instance, if you have 2 tools that produce source maps and you call one with
     the result of the other.
 
 [closure]: http://code.google.com/p/closure-compiler/wiki/SourceMaps
 [spec]: https://docs.google.com/a/google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit