Replace NO-BREAK SPACE (U+00A0) with normal space in markdown

When I used pandoc to automatically convert old html specs to markdown, unfortunately it converted some spaces to unicode NO-BREAK SPACE (U+00A0, utf-8 encoding c2 a0) instead of normal space. They look like normal space by most text editors. In all cases, a real non-breaking space was not needed. If so,   should be used instead.