Cleaned up the version.h markdown.

doc/api/mruby/version.h.md

-#### Macros
-### MRUBY_RUBY_VERSION
+### Macros
+#### MRUBY_RUBY_VERSION
 The version of Ruby used by mruby.
-### MRUBY_RUBY_ENGINE
+#### MRUBY_RUBY_ENGINE
 Ruby engine.
-### MRUBY_VERSION
+#### MRUBY_VERSION
 The mruby version.
-### MRUBY_RELEASE_MAJOR
+#### MRUBY_RELEASE_MAJOR
 Major release version.
-### MRUBY_RELEASE_MINOR
+#### MRUBY_RELEASE_MINOR
 Minor release version.
-### MRUBY_RELEASE_NO
+#### MRUBY_RELEASE_NO
 Release number.
-### MRUBY_RELEASE_DATE
+#### MRUBY_RELEASE_DATE
 Release date as a string.
-### MRUBY_RELEASE_YEAR
+#### MRUBY_RELEASE_YEAR
 Release year.
-### MRUBY_RELEASE_MONTH
+#### MRUBY_RELEASE_MONTH
 Release month.
-### MRUBY_RELEASE_DAY
+#### MRUBY_RELEASE_DAY
 Release day.
-### MRUBY_BIRTH_YEAR
+#### MRUBY_BIRTH_YEAR
 The year mruby was first created.
-### MRUBY_AUTHOR
+#### MRUBY_AUTHOR
 Mruby's authors.
-### MRB_STRINGIZE0(expr)
+#### MRB_STRINGIZE0(expr)
 A passed in expression.
-### MRB_STRINGIZE(expr)
+#### MRB_STRINGIZE(expr)
 Passes in an expression to MRB_STRINGIZE0.
-### MRUBY_DESCRIPTION
+#### MRUBY_DESCRIPTION
 mruby's version, and release date.
-### MRUBY_COPYRIGHT
+#### MRUBY_COPYRIGHT
 mruby's copyright information.