rdoc: fix syntax highlighting

rdoc.md

@@ -6,6 +6,7 @@ layout: 2017/sheet
 
 ### Basic RDoc format
 
+```rb
 # Foo.
 #
 # @example
@@ -18,16 +19,17 @@ layout: 2017/sheet
 # @see http://url.com
 #
 # @return [true] if so
+```
 
 ### Inline
 
-```
+```markdown
 *bold*
 _emphasized_
 +code+
 ```
 
-```
+```markdown
 http://www.link.com
 See Models::User@Examples
 {Google}[http://google.com]
@@ -45,14 +47,14 @@ module MyModule # :nodoc: all
 
 ### Definition lists
 
-```
+```rb
 # == Definition lists
 #
 # list::  hi.
 # +foo+:: parameterized
 ```
 
-```
+```rb
 # == Definition lists
 # [foo]   also
 # [bar]   like this
@@ -60,11 +62,14 @@ module MyModule # :nodoc: all
 
 ### Return types
 
+```rb
 # @return [String]
 # @return [String, nil] the name
+```
 
 ### Callseq
 
+```rb
 # :call-seq:
 #   ARGF.readlines(sep=$/)     -> array
 #   ARGF.readlines(limit)      -> array
@@ -73,10 +78,13 @@ module MyModule # :nodoc: all
 #   ARGF.to_a(sep=$/)     -> array
 #   ARGF.to_a(limit)      -> array
 #   ARGF.to_a(sep, limit) -> array
+```
 
 ### Category
 
+```rb
 # :category: Utilities
+```
 
 ### Sections
 
@@ -91,11 +99,11 @@ def expired?
 
 ### Using tomdoc
 
-```
+```rb
 # :markup: TomDoc
 ```
 
-Plase this at the beginning of the file.
+Place this at the beginning of the file.
 
 ## Also see
 {: .-one-column}