Fix links in 1990/theorem/README.md

1990/theorem/index.html

@@ -416,7 +416,7 @@ Location: <a href="../../location.html#US">US</a> - <em>United States of America
 <h2 id="judges-remarks">Judges’ remarks:</h2>
 <p>The program’s source implements four functions, all from the
 same source file!</p>
-<p>When you compile <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/1990/theorem/Rtheorem.c">theorem.c</a> as is and run with 5 args, it
+<p>When you compile <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/1990/theorem/theorem.c">theorem.c</a> as is and run with 5 args, it
 numerically solves the equation <code>y'=f(x,y)</code>, with a step size of <code>h</code> (see above
 usage), over the interval <code>x=[x1,x2]</code>, with the initial condition of <code>y(x1)=y1</code>.</p>
 <p>The ‘expression’ <code>f(x,y)</code> is any function of <code>x</code> and <code>y</code> with the