Respect the directory structure of `include`

As mentioned in the README, two files have been put under the `mruby`
directory.

doc/api/README.md

@@ -8,23 +8,23 @@ Header name|Features
 -----------|--------
 [mrbconf.h](../mrbconf/README.md)|Defines macros for mruby configurations.
 [mruby.h](./mruby.h.md)|Main header of mruby C API. Include this first.
-[mruby/array.h](./mruby.array.h.md)|`Array` class.
-[mruby/class.h](./mruby.class.h.md)|`Class` class.
-[mruby/compile.h](./mruby.compile.h.md)|mruby compiler.
-[mruby/data.h](./mruby.data.h.md)|User defined object.
-[mruby/debug.h](./mruby.debug.h.md)|Debugging.
-[mruby/dump.h](./mruby.dump.h.md)|Dumping compiled mruby script.
-[mruby/error.h](./mruby.error.h.md)|Error handling.
-[mruby/gc.h](./mruby.gc.h.md)|Uncommon memory management stuffs.
-[mruby/hash.h](./mruby.hash.h.md)|`Hash` class.
-[mruby/irep.h](./mruby.irep.h.md)|Compiled mruby script.
-[mruby/khash.h](./mruby.khash.h.md)|Defines of khash which is used in hash table of mruby.
-[mruby/numeric.h](./mruby.numeric.h.md)|`Numeric` class and sub-classes of it.
-[mruby/opode.h](./mruby.opcode.h.md)|Operation codes used in mruby VM.
-[mruby/proc.h](./mruby.proc.h.md)|`Proc` class.
-[mruby/range.h](./mruby.range.h.md)|`Range` class.
-[mruby/re.h](./mruby.re.h.md)|`Regexp` class.
-[mruby/string.h](./mruby.string.h.md)|`String` class.
-[mruby/value.h](./mruby.value.h.md)|`mrb_value` functions and macros.
-[mruby/variable.h](./mruby.variable.h.md)|Functions to access to mruby variables.
-[mruby/version.h](./mruby.version.h.md)|Macros of mruby version.
+[mruby/array.h](./mruby/array.h.md)|`Array` class.
+[mruby/class.h](./mruby/class.h.md)|`Class` class.
+[mruby/compile.h](./mruby/compile.h.md)|mruby compiler.
+[mruby/data.h](./mruby/data.h.md)|User defined object.
+[mruby/debug.h](./mruby/debug.h.md)|Debugging.
+[mruby/dump.h](./mruby/dump.h.md)|Dumping compiled mruby script.
+[mruby/error.h](./mruby/error.h.md)|Error handling.
+[mruby/gc.h](./mruby/gc.h.md)|Uncommon memory management stuffs.
+[mruby/hash.h](./mruby/hash.h.md)|`Hash` class.
+[mruby/irep.h](./mruby/irep.h.md)|Compiled mruby script.
+[mruby/khash.h](./mruby/khash.h.md)|Defines of khash which is used in hash table of mruby.
+[mruby/numeric.h](./mruby/numeric.h.md)|`Numeric` class and sub-classes of it.
+[mruby/opode.h](./mruby/opcode.h.md)|Operation codes used in mruby VM.
+[mruby/proc.h](./mruby/proc.h.md)|`Proc` class.
+[mruby/range.h](./mruby/range.h.md)|`Range` class.
+[mruby/re.h](./mruby/re.h.md)|`Regexp` class.
+[mruby/string.h](./mruby/string.h.md)|`String` class.
+[mruby/value.h](./mruby/value.h.md)|`mrb_value` functions and macros.
+[mruby/variable.h](./mruby/variable.h.md)|Functions to access to mruby variables.
+[mruby/version.h](./mruby/version.h.md)|Macros of mruby version.