minor documentation fixes (#197)

* "is an an" -> "is an"

* grammar fix

* grammar fix

* update HISTORY.md

HISTORY.md

@@ -6,7 +6,11 @@
  * Handle SpSubview_col and SpSubview_row in Armadillo 9.870
     ([#194](https://github.com/mlpack/ensmallen/pull/194)).
 
- * ...
+ * Minor documentation fixes
+    ([#197](https://github.com/mlpack/ensmallen/pull/197)).
+
+
+
 
 ### ensmallen 2.12.0: "Stir Crazy"
 ###### 2020-03-28

doc/callbacks.md

@@ -212,7 +212,7 @@ std::cout << "The optimized model found by AdaDelta has the "
 
 ## Callback States
 
-Callbacks are called at different states during the optimization process:
+Callbacks are called at several states during the optimization process:
 
 * At the beginning and end of the optimization process.
 * After any call to `Evaluate()` and `EvaluateConstraint`.

doc/optimizers.md

@@ -179,7 +179,7 @@ optimizer.Optimize(f, coordinates);
 
 *An optimizer for [differentiable separable functions](#differentiable-separable-functions).*
 
-Adam is an an algorithm for first-order gradient-based optimization of
+Adam is an algorithm for first-order gradient-based optimization of
 stochastic objective functions, based on adaptive estimates of lower-order
 moments.
 
@@ -1914,7 +1914,7 @@ optimizer.Optimize(f, coordinates);
 
 QHAdam is an optimizer that uses quasi-hyperbolic descent with the Adam
 optimizer.  This replaces the moment estimators of Adam with quasi-hyperbolic
-terms, and different values of the `v1` and `v2` parameters are equivalent to
+terms, and various values of the `v1` and `v2` parameters are equivalent to
 the following other optimizers:
 
  * When `v1 = v2 = 1`, `QHAdam` is equivalent to `Adam`.