Correct user namespace from `cust_` to `custom_`

docs/strategy-advanced.md

@@ -15,7 +15,7 @@ If you're just getting started, please be familiar with the methods described in
 
 Storing information can be accomplished by creating a new dictionary within the strategy class.
 
-The name of the variable can be chosen at will, but should be prefixed with `cust_` to avoid naming collisions with predefined strategy variables.
+The name of the variable can be chosen at will, but should be prefixed with `custom_` to avoid naming collisions with predefined strategy variables.
 
 ```python
 class AwesomeStrategy(IStrategy):

docs/strategy-callbacks.md

@@ -43,7 +43,7 @@ class AwesomeStrategy(IStrategy):
         if self.config['runmode'].value in ('live', 'dry_run'):
             # Assign this to the class by using self.*
             # can then be used by populate_* methods
-            self.cust_remote_data = requests.get('https://some_remote_source.example.com')
+            self.custom_remote_data = requests.get('https://some_remote_source.example.com')
 
 ```