fix formatting

src/2_debye_model.md

@@ -186,7 +186,7 @@ $$
 
 !!! note "Integral over $k$-space"
 
-  This conversion from a sum over a discrete grid of $k$-space states to a volume integral is one of the extremely commonly used ideas in solid state physics: it provides us a way to count all the possible waves.
+    This conversion from a sum over a discrete grid of $k$-space states to a volume integral is one of the extremely commonly used ideas in solid state physics: it provides us a way to count all the possible waves.
 
 We can use this approximation and rewrite the total energy as an integral:
 $$