Correcting a spelling mistake and slight clarification of newly added (en) lang strings, specifically for the CSS optimiser.

GitHub code:
https://github.com/sdc/moodle/commit/c0b2609903d928e3562a020879fa8587cb460594

Patch:
diff --git a/lang/en/admin.php b/lang/en/admin.php
index 42dbcf3..41629f6 100644
— a/lang/en/admin.php
+++ b/lang/en/admin.php
@@ -468,7 +468,7 @@
$string['enablecomments'] = 'Enable comments';
$string['enablecourserequests'] = 'Enable course requests';
$string['enablecssoptimiser'] = 'Enable CSS optimiser';
-$string['enablecssoptimiser_desc'] = 'When enabled CSS will be run through an optimisation process before being cached. The optimiser processes the CSS removing duplicate rules and styles, as well as white space removeable and reformatting. Please note turning this on at the same time as theme designer mode is aweful for performance but will help theme designers create optimised CSS.';
+$string['enablecssoptimiser_desc'] = 'When enabled CSS will be run through an optimisation process before being cached. The optimiser processes the CSS removing duplicate rules and styles, as well as removing white space and reformatting. Please note: turning this on at the same time as theme designer mode is awful for performance but will help theme designers create optimised CSS.';
$string['enabledevicedetection'] = 'Enable device detection';
$string['enablegravatar'] = 'Enable Gravatar';
$string['enablegravatar_help'] = 'When enabled Moodle will attempt to fetch a user profile picture from Gravatar if the user has not uploaded an image.';