docs: Fix a few typos

There are small typos in:
- README.rst
- pyramid_multiauth/__init__.py

Fixes:
- Should read `processed` rather than `procecesed`.
- Should read `previously` rather than `previusly`.
- Should read `introspect` rather than `instrospect`.

Signed-off-by: Tim Gates <tim.gates@iress.com>

README.rst

@@ -74,7 +74,7 @@ module into your configurator::
 In this example you would get a MultiAuthenticationPolicy with three stacked
 auth policies.  The first two, ipauth1 and ipauth2, are defined as the name of
 of a callable along with a set of keyword arguments.  The third is defined as
-the name of a module, pyramid_browserid, which will be procecesed via the
+the name of a module, pyramid_browserid, which will be processed via the
 standard config.include() mechanism.
 
 The end result would be a system that authenticates users via BrowserID, and

pyramid_multiauth/__init__.py

@@ -146,7 +146,7 @@ def remember(self, request, principal, **kw):
         return headers
 
     def forget(self, request):
-        """Forget a previusly remembered userid.
+        """Forget a previously remembered userid.
 
         This method returns the concatenation of the headers returned by each
         authn policy.
@@ -160,7 +160,7 @@ def get_policies(self):
         """Get the list of contained authentication policies, as tuple of
         name and instances.
 
-        This may be useful to instrospect the configured policies, and their
+        This may be useful to introspect the configured policies, and their
         respective name defined in configuration.
         """
         return [(getattr(policy, "_pyramid_multiauth_name", None), policy)