Fix a symbol error

There is a wrong use ' here, which causes the document
to make errors when copying the code for
experimentation.

Change-Id: I3ae19f591ffd668bcd1563bd1e317b21b799dba6

doc/source/user/persistence.rst

@@ -129,7 +129,7 @@ Using this function to fetch a backend might look like:
 
     ...
     persistence = backends.fetch(conf={
-        "connection': "mysql",
+        "connection": "mysql",
         "user": ...,
         "password": ...,
     })