Modify README for examples

mrbgems/g/clib_example/README.md

-CLib Extension Example
+C Extension Example
 =========
 
 This is an example gem which implements a C extension.

mrbgems/g/mrblib_example/README.md

-hello world
+Pure Ruby Extension Example
 =========
 
-This is an example gem which implements just one method +HW.say+ in a C extension.
+This is an example gem which implements a pure Ruby extension.