fix: Update 7.1.1数学基础.md (#28)

fix error

docs/foxy/chapt7/7.1.1数学基础.md

@@ -155,7 +155,7 @@ c_{21} = 3*1+4*0 = 3
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-c_{22} = 3+0 + 4*3 = 12 
+c_{22} = 3*0 + 4*3 = 12 
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