MX4 Typo in row swap activity (#423)

source/linear-algebra/source/04-MX/04.ptx

@@ -84,7 +84,7 @@ left-multiplying? (<m>2R_3\to R_3</m>)
     <p>
 Which of these tweaks of the identity matrix
 yields a matrix that swaps the second and third rows of <m>A</m>
-when left-multiplying? (<m>R_1\leftrightarrow R_3</m>)
+when left-multiplying? (<m>R_2\leftrightarrow R_3</m>)
   <me>
    \left[\begin{array}{ccc} \unknown &amp; \unknown &amp; \unknown \\ \unknown &amp; \unknown &amp; \unknown \\ \unknown &amp; \unknown &amp; \unknown \end{array}\right]
    \left[\begin{array}{ccc} 2 &amp; 7 &amp; -1 \\ 0 &amp; 3 &amp; 2 \\ 1 &amp; 1 &amp; -1 \end{array}\right]