Update textbook.fr.md

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

@@ -40,7 +40,7 @@ $`x^0=ct\;;\;x^1=x\;;\;x^2=y\;;\;x^3=z`$
 
 $`\begin{align}
 ds^2 &=(dx^0)^2 -(dx^1)^2 -(dx^2)^2 -(dx^3)^2\\
-&=(dx'^0)^2 -(dx'^1)^2 -(dx'^)^2 -(dx'^3)^2\\
+&=(dx^0)^2 -(dx^1)^2 -(dx^2)^2 -(dx^3)^2\\
 \end{align}`$
 
 $`\begin{align}