Fix typo (#1262)

Author: davidbrochart

docs/source/operators/multiple-extensions.rst

@@ -7,7 +7,7 @@ Managing multiple extensions
 One of the major benefits of Jupyter Server is that you can run serve multiple
 Jupyter frontend applications above the same Tornado web server.
 That's because every Jupyter frontend application is now a server extension.
-When you run a Jupyter Server will multiple extensions enabled, each extension
+When you run a Jupyter Server with multiple extensions enabled, each extension
 appends its own set of handlers and static assets to the server.
 
 Listing extensions