message: Correct doc of Ftrl optimizer by remplacing $ by $$
diff: @@ -35,18 +35,18 @@ class Ftrl(optimizer_v2.OptimizerV2):
  loss function).
  Initialization:
  - $t = 0$
  - $n_{0} = 0$
  - $\sigma_{0} = 0$
  - $z_{0} = 0$
  -
  - Update ($i$ is variable index):
  - $t = t + 1$
  - $n_{t,i} = n_{t-1,i} + g_{t,i}^{2}$
  - $\sigma_{t,i} = (\sqrt{n_{t,i}} - \sqrt{n_{t-1,i}}) / \alpha$
  - $z_{t,i} = z_{t-1,i} + g_{t,i} - \sigma_{t,i} * w_{t,i}$
  - $w_{t,i} = - ((\beta+\sqrt{n+{t}}) / \alpha + \lambda_{2})^{-1} * (z_{i} -
  - sgn(z_{i}) * \lambda_{1}) if \abs{z_{i}} > \lambda_{i} else 0$
  + $$t = 0$$
  + $$n_{0} = 0$$
  + $$\sigma_{0} = 0$$
  + $$z_{0} = 0$$
  +
  + Update ($$i$$ is variable index):
  + $$t = t + 1$$
  + $$n_{t,i} = n_{t-1,i} + g_{t,i}^{2}$$
  + $$\sigma_{t,i} = (\sqrt{n_{t,i}} - \sqrt{n_{t-1,i}}) / \alpha$$
  + $$z_{t,i} = z_{t-1,i} + g_{t,i} - \sigma_{t,i} * w_{t,i}$$
  + $$w_{t,i} = - ((\beta+\sqrt{n+{t}}) / \alpha + \lambda_{2})^{-1} * (z_{i} -
  + sgn(z_{i}) * \lambda_{1}) if \abs{z_{i}} > \lambda_{i} else 0$$
  Check the documentation for the l2_shrinkage_regularization_strength
  parameter for more details when shrinkage is enabled, where gradient is
  