Use double quotes in html_renderer.cr, begin_code (crystal-lang#5701)

* Use double quotes in html_renderer.cr, begin_code

It should use double quotes there instead of apostrophes. Because thats generating bad html code.
For example this is what glitch.com (glitch is an online html editor) says to an markdown crystal code block:
https://imgur.com/a/6nUKz
And other sources say too that double quotes are better.

* Update markdown_spec.cr

* Update markdown_spec.cr

* Update markdown_spec.cr

spec/std/markdown/markdown_spec.cr

@@ -67,7 +67,7 @@ describe Markdown do
   assert_render "    Hello\n   World", "<pre><code>Hello</code></pre>\n\n<p>World</p>"
   assert_render "    Hello\n\n\nWorld", "<pre><code>Hello</code></pre>\n\n<p>World</p>"
 
-  assert_render "```crystal\nHello\nWorld\n```", "<pre><code class='language-crystal'>Hello\nWorld</code></pre>"
+  assert_render "```crystal\nHello\nWorld\n```", %(<pre><code class="language-crystal">Hello\nWorld</code></pre>)
   assert_render "Hello\n```\nWorld\n```", "<p>Hello</p>\n\n<pre><code>World</code></pre>"
   assert_render "```\n---\n```", "<pre><code>---</code></pre>"
 

src/markdown/html_renderer.cr

@@ -54,7 +54,7 @@ class Markdown::HTMLRenderer
     if language.nil?
       @io << "<pre><code>"
     else
-      @io << "<pre><code class='language-#{language}'>"
+      @io << %(<pre><code class="language-#{language}">)
     end
   end