Update README.md

Fix mrbgems doc link.

README.md

@@ -52,7 +52,7 @@ Or
 
 mruby contains a package manager called *mrbgems*. To create extensions
 in C and/or Ruby you should create a *GEM*. For a documentation of how to
-use mrbgems consult the file [doc/mrbgems/README.md](doc/mrbgems/README.md). For example code of
+use mrbgems consult the file [doc/guides/mrbgems.md](doc/guides/mrbgems.md). For example code of
 how to use mrbgems look into the folder *examples/mrbgems/*.
 
 ## License