Fix footnote label in chapter-4-algorithms.md

Rename footnote identifier from [^operation costs] to [^operation-costs] to match the reference and ensure the footnote link resolves correctly in the algorithms chapter.

Author: sean-parent

better-code/src/chapter-4-algorithms.md

@@ -253,7 +253,7 @@ magnitude[^operation-costs]:
 | 10^3 | Kernel call, thread context switch (direct costs), exception thrown and caught |
 | 10^4 | Thread context switch (including cache invalidation) |
 
-[^operation costs]: [_Infographics: Operation Costs in CPU Clock
+[^operation-costs]: [_Infographics: Operation Costs in CPU Clock
     Cycles_](http://ithare.com/infographics-operation-costs-in-cpu-clock-cycles/)
 
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