Fix some minor spelling and grammar in documentation

mongoengine/base/fields.py

@@ -43,7 +43,7 @@ class BaseField(object):
         :param required: If the field is required. Whether it has to have a
             value or not. Defaults to False.
         :param default: (optional) The default value for this field if no value
-            has been set (or if the value has been unset).  It Can be a
+            has been set (or if the value has been unset).  It can be a
             callable.
         :param unique: Is the field value unique or not.  Defaults to False.
         :param unique_with: (optional) The other field this field should be

mongoengine/dereference.py

@@ -12,7 +12,7 @@ class DeReference(object):
     def __call__(self, items, max_depth=1, instance=None, name=None):
         """
         Cheaply dereferences the items to a set depth.
-        Also handles the convertion of complex data types.
+        Also handles the conversion of complex data types.
 
         :param items: The iterable (dict, list, queryset) to be dereferenced.
         :param max_depth: The maximum depth to recurse to