Fix several docstrings (#451)

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- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [N/A] I have added a one-line description of my change to the
changelog in
      `HISTORY.md`
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ribs/emitters/_gradient_operator_emitter.py

@@ -67,7 +67,7 @@ class GradientOperatorEmitter(EmitterBase):
         sigma_g (float): Step size used for gradient arborescence in ask(),
             branching from the parents generated by ask_dqd(). If measure
             gradients are used, this acts as the standard deviation of the
-            Gaussian from which to sample the step size.Other wise, this acts as
+            Gaussian from which to sample the step size. Otherwise, this acts as
             the step size itself.
         initial_solutions (array-like): An (n, solution_dim) array of solutions
             to be used when the archive is empty. If this argument is None, then

ribs/emitters/operators/_iso_line.py

@@ -12,7 +12,7 @@ class IsoLineOperator(OperatorBase):
 
     Args:
         iso_sigma (float): Scale factor for the isotropic distribution used to
-                generate solutions.
+            generate solutions.
         line_sigma (float): Scale factor for the line distribution used when
             generating solutions.
         sigma (float or array-like): Standard deviation of the Gaussian
@@ -48,10 +48,10 @@ def ask(self, parents):
         """ Adds Isotropic Guassian noise and directional noise to parents.
 
         Args:
-            parents (array-like): (2, batch_size, solution_dim)
-                parents[0] array of solutions selected by emitter
-                parents[1] array of second batch of solutions passed by
-                  emitter. Used for calculating directional correlation.
+            parents (array-like): (2, batch_size, solution_dim) parents[0] array
+                of solutions selected by emitter parents[1] array of second
+                batch of solutions passed by emitter. Used for calculating
+                directional correlation.
 
         Returns:
             numpy.ndarray: ``(batch_size, solution_dim)`` array that contains

ribs/emitters/operators/_operator_base.py

@@ -14,9 +14,8 @@ def ask(self, parents):
         """Operates on parents to generate new solutions.
 
         Args:
-            parents (array-like): (batch_size, solution_dim) array of
-                solutions to be mutated. Typically passed in by an emitter
-                after selection from an archive.
+            parents (array-like): Array of solutions to be mutated. Typically
+                passed in by an emitter after selection from an archive.
 
         Returns:
             numpy.ndarray: ``(batch_size, solution_dim)`` array that contains