Update notes/matrices.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: djeada

notes/matrices.md

@@ -664,7 +664,7 @@ r_{\min}(s)=\max\!\big(0,\ s-(C-1)\big),\qquad
 r_{\max}(s)=\min\!\big(R-1,\ s\big).
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-The diagonal set is $\{(r,,s-r)\mid r_{\min}(s)\le r\le r_{\max}(s)\}$.
+The diagonal set is $\{(r,s-r)\mid r_{\min}(s)\le r\le r_{\max}(s)\}$.
 Traverse with alternating direction:
 
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