docs: dev/feature-flags: fix a few typos

docs/dev/feature-flags.rst

@@ -5,7 +5,7 @@ Feature flags provide a convenient way to define package selections without
 making it necessary to list each package explicitly.
 
 The main feature flag definition file is ``package/features``, but each package
-feed can provide addition defintions in a file called ``features`` at the root
+feed can provide additional defintions in a file called ``features`` at the root
 of the feed repository.
 
 Each flag *$flag* without any explicit definition will simply include the package
@@ -35,7 +35,7 @@ This will
 * Disable the inclusion of a (non-existent) package called *gluon-web-wizard*
 * Enable three config mode packages when the *web-wizard* feature is enabled
 * Enable *gluon-config-mode-mesh-vpn* when both *web-wizard* and one
-  of *mesh-vpn-fastd* and *mesh-vpn-tunneldigger* is enabled
+  of *mesh-vpn-fastd* and *mesh-vpn-tunneldigger* are enabled
 
 Supported syntax elements of logical expressions are: