updated historical terms with modern equivalents

### Description
There were a few remnant uses of master/slave in the code and docs. The project previously made a decision to move away from them to use modern and inclusive terminology.

This PR does not cover a bug or necessitate a documented entry into the changelog, so an issue ticket was not created.

### Checklist
This pull request is:

- [x] A documentation / typographical error fix
- [x] A short code fix
- [ ] A new feature implementation

Closes: #5381
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5381
Pull-request-sha: 92597e83d0e1e18960dbb39b604b313e7a1cbb30

Change-Id: I1fb34fe5ab6c19fd7360568d7b51cdea9d271b3b

1 files changed, 7 insertions, 7 deletions

diff --git a/doc/build/orm/persistence_techniques.rst b/doc/build/orm/persistence_techniques.rst
index 18e4d1269..27c8c382f 100644
--- a/doc/build/orm/persistence_techniques.rst
+++ b/doc/build/orm/persistence_techniques.rst
@@ -591,21 +591,21 @@ More comprehensive rule-based class-level partitioning can be built by
 overriding the :meth:`.Session.get_bind` method.   Below we illustrate
 a custom :class:`.Session` which delivers the following rules:
 
-1. Flush operations are delivered to the engine named ``master``.
+1. Flush operations are delivered to the engine named ``leader``.
 
 2. Operations on objects that subclass ``MyOtherClass`` all
    occur on the ``other`` engine.
 
 3. Read operations for all other classes occur on a random
-   choice of the ``slave1`` or ``slave2`` database.
+   choice of the ``follower1`` or ``follower2`` database.
 
 ::
 
     engines = {
-        'master':create_engine("sqlite:///master.db"),
+        'leader':create_engine("sqlite:///leader.db"),
         'other':create_engine("sqlite:///other.db"),
-        'slave1':create_engine("sqlite:///slave1.db"),
-        'slave2':create_engine("sqlite:///slave2.db"),
+        'follower1':create_engine("sqlite:///follower1.db"),
+        'follower2':create_engine("sqlite:///follower2.db"),
     }
 
     from sqlalchemy.orm import Session, sessionmaker
@@ -616,10 +616,10 @@ a custom :class:`.Session` which delivers the following rules:
             if mapper and issubclass(mapper.class_, MyOtherClass):
                 return engines['other']
             elif self._flushing:
-                return engines['master']
+                return engines['leader']
             else:
                 return engines[
-                    random.choice(['slave1','slave2'])
+                    random.choice(['follower1','follower2'])
                 ]
 
 The above :class:`.Session` class is plugged in using the ``class_``