Update textbook.fr.md

12.temporary_ins/44.relativity/40.n4/10.special-relativity/07.coherence/textbook.fr.md

@@ -44,7 +44,7 @@ ds^2 &=(dx^0)^2 -(dx^1)^2 -(dx^2)^2 -(dx^3)^2\\
 \end{align}`$
 
 $`\begin{align}
-ds^2 &=\big(dx^0\big'\big)^2 -\big(dx^1\big)^2 -\big(dx^2\big)^2 -\big(dx^3\big)^2 \\
+ds^2 &=\big(dx^0 \big' \big)^2 -\big(dx^1\big\'\big)^2 -\big(dx^2\big)^2 -\big(dx^3\big)^2 \\
 & \\
 &=g_{ab}\;dx^a dx^b
 \end{align}`$