Fix rendering bug of Markdown Code.

When a code block included a ```, it would not be rendered correctly.

(cherry picked from commit 60eda14)
ref #14638

base/markdown/render/plain.jl

@@ -20,9 +20,12 @@ function plain{l}(io::IO, header::Header{l})
 end
 
 function plain(io::IO, code::Code)
-    println(io, "```", code.language)
+    # If the code includes a fenced block this will break parsing,
+    # so it must be enclosed by a longer ````-sequence.
+    n = mapreduce(length, max, 2, matchall(r"^`+"m, code.code)) + 1
+    println(io, "`" ^ n, code.language)
     println(io, code.code)
-    println(io, "```")
+    println(io, "`" ^ n)
 end
 
 function plain(io::IO, p::Paragraph)

test/markdown.jl

@@ -41,6 +41,14 @@ foo
 @test md"``code```more code``" == MD(Any[Paragraph(Any[Code("","code```more code")])])
 @test md"``code``````more code``" == MD(Any[Paragraph(Any[Code("","code``````more code")])])
 
+code_in_code = md"""
+````
+```
+````
+"""
+@test code_in_code == MD(Code("```"))
+@test plain(code_in_code) == "````\n```\n````\n"
+
 @test md"""
 * one
 * two