add missing dot

Author: mantepse

src/sage/rings/invariants/invariant_theory.py

@@ -7,7 +7,7 @@
 degree `d` in `n` variables. The special linear group `SL(n,\CC)` acts
 on the variables `(x_1,\dots, x_n)` linearly,
 
-. MATH::
+.. MATH::
 
     (x_1,\dots, x_n)^t \to A (x_1,\dots, x_n)^t
     ,\qquad