Fix a document in doc/mrbgems/README.md

doc/mrbgems/README.md

@@ -63,7 +63,7 @@ This will cause the *custom* GemBox to be read in during the build process,
 adding *mruby-time* and *mrbgems-example* to the build.
 
 If you want, you can put GemBox outside of mruby directory. In that case you must
-specify absolute path like below.
+specify an absolute path like below.
 ```ruby
 conf.gembox "#{ENV["HOME"]}/mygemboxes/custom"
 ```