CONTENTS: adjusted to current state and tuned formatting

This commit renames file CONTENT to CONTENTS, reflecting its
intended meaning. For the difference between the two words, see
http://english.stackexchange.com/questions/13556/content-or-contents

CONTENT

-CONTENT of the repository:
-
-* README                                                   the README file, which contains all important information
-* COPYING.MIT                                              MIT license
-* conf/layer.conf                                          configuration file for this layer
-* classes/catkin.bbclass                                   class for catkin-based packages
-* recipes-core/images                                      recipes of images for native compilation of ROS
-* recipes-devtools                                         recipes of system prerequisites of ROS
-* recipes-ros                                              recipes for cross-compiling ROS packages
-

CONTENTS

+CONTENTS of the repository:
+
+* README                    the README file with all important information
+* COPYING.MIT               MIT license
+* conf/layer.conf           configuration file for this layer
+* classes/catkin.bbclass    class for catkin-based packages
+* recipes-core/images       recipes of images for using ROS
+* recipes-devtools          recipes of system prerequisites of ROS
+* recipes-ros               recipes for cross-compiling ROS packages