Fix typos

Co-authored-by: Paul Moore <p.f.moore@gmail.com>

Author: astrofrog

peps/pep-0771.rst

@@ -151,15 +151,15 @@ extras are not installed. Otherwise, the default extras are used.
 
 For example, if a package
 defines an ``extra1`` default extra as well as a non-default ``extra2``
-extra, then if a user was to install the package with::
+extra, then if a user were to install the package with::
 
     pip install package
 
 the ``extra1`` dependency would be included. If the user instead uses::
 
     pip install package[extra2]
 
-then the ``extra1`` extras would not be installed.
+then the ``extra1`` extra would not be installed.
 
 If the same package is specified multiple times in an installation command or
 dependency tree, the default extras must be installed if any of the instances of
@@ -180,7 +180,7 @@ In some cases, package maintainers may want to facilitate installing packages
 without any default extras. In this case, as will be shown in more detail in
 `How to teach this`_, the best approach is to define an extra which could be
 called e.g. ``minimal`` or ``nodefault`` (the naming would be up to the package
-maintainer) which would be an empty set of dependencies. If this extras is
+maintainer) which would be an empty set of dependencies. If this extra is
 specified, no default extras will be included, so that e.g. ``package[minimal]``
 would include only required dependencies and no extras. Note that this requires
 no additional specification and is a natural consequence of the rule described