Fix typos in backend documentation

Author: kyleam

pymc3/backends/__init__.py

@@ -100,8 +100,7 @@
 - point: Returns values for each variable at a single iteration. This is
   called if the backend is indexed with a single integer.
 
-- __len__: This should return the number of draws (for the highest chain
-  number).
+- __len__: This should return the number of draws.
 
 When `pymc3.sample` finishes, it wraps all trace objects in a MultiTrace
 object that provides a consistent selection interface for all backends.

pymc3/backends/base.py

@@ -212,7 +212,7 @@ def get_values(self, varname, burn=0, thin=1, combine=False, chains=None,
             If True, results from `chains` will be concatenated.
         chains : int or list of ints
             Chains to retrieve. If None, all chains are used. A single
-            values can also accepted.
+            chain value can also be given.
         squeeze : bool
             Return a single array element if the resulting list of
             values only has one element. If False, the result will

pymc3/backends/sqlite.py

@@ -8,7 +8,7 @@
 
  recid (INT), draw (INT), chain (INT),  v1 (FLOAT), v2 (FLOAT), v3 (FLOAT) ...
 
-The variable column names are extended to reflect addition dimensions.
+The variable column names are extended to reflect additional dimensions.
 For example, a variable with the shape (2, 2) would be stored as
 
  key (INT), draw (INT), chain (INT),  v1_1 (FLOAT), v1_2 (FLOAT), v2_1 (FLOAT) ...