Unify the uses of "file system" vs "fileystem".

Use "filesystem", which is more prevailing in the source code.

docs/quickstart.rst

@@ -222,7 +222,7 @@ The following converters exist:
    Though they look rather similar, they differ in their use of the trailing
    slash in the URL *definition*.  In the first case, the canonical URL for the
    `projects` endpoint has a trailing slash.  In that sense, it is similar to
-   a folder on a file system.  Accessing it without a trailing slash will cause
+   a folder on a filesystem.  Accessing it without a trailing slash will cause
    Flask to redirect to the canonical URL with the trailing slash.
 
    In the second case, however, the URL is defined without a trailing slash,

flask/app.py

@@ -85,7 +85,7 @@ class Flask(_PackageBoundObject):
 
         The idea of the first parameter is to give Flask an idea of what
         belongs to your application.  This name is used to find resources
-        on the file system, can be used by extensions to improve debugging
+        on the filesystem, can be used by extensions to improve debugging
         information and a lot more.
 
         So it's important what you provide there.  If you are using a single