fix HTML error in 2013/birken/index.html

The HTML error in `2013/birken/index.html` introduced by
mistake in commit 9b55039fd4.

2013/birken/README.md

@@ -271,7 +271,6 @@ contains at least 1 square, like the red square below, then it is possible to
 construct a flat platform above it for the generation of the next row:
 
 <img src="platform2.gif"
- alt="image of a tattoo of the 1984 anonymous C code"
  alt="animated gif showing a bottom Tetris row with a red square and constucted patterns above it"
  width=528 height=156>
 

2013/birken/index.html

@@ -331,7 +331,6 @@ down to that first <code>T</code>. Meaning, as long as the previously generated
 contains at least 1 square, like the red square below, then it is possible to
 construct a flat platform above it for the generation of the next row:</p>
 <p><img src="platform2.gif"
- alt="image of a tattoo of the 1984 anonymous C code"
  alt="animated gif showing a bottom Tetris row with a red square and constucted patterns above it"
  width=528 height=156></p>
 <p>In the middle of platform construction, the bottom row gets completed and