HTML reader: Removed "button" from block tag list.

It is already in the eitherBlockOrInlineTag list, and
should be both places.

Closes #3717.

Note: the result of this change is that there will be
p tags around the whole paragraph.  That is the right
result, because the `button` tags are treated as inline
HTML here, and the whole chunk of text is a Markdown
paragraph.

src/Text/Pandoc/Readers/HTML.hs

@@ -886,7 +886,7 @@ inlineHtmlTags = ["a", "abbr", "acronym", "b", "basefont", "bdo", "big",
 
 blockHtmlTags :: [String]
 blockHtmlTags = ["?xml", "!DOCTYPE", "address", "article", "aside",
-                 "blockquote", "body", "button", "canvas",
+                 "blockquote", "body", "canvas",
                  "caption", "center", "col", "colgroup", "dd", "details",
                  "dir", "div",
                  "dl", "dt", "fieldset", "figcaption", "figure",