Update _extras/guide.md

Co-authored-by: Steve Crouch <s.crouch@software.ac.uk>

Author: anenadic

_extras/guide.md

@@ -90,7 +90,7 @@ when students start with group exercises.
 At the workshop, when using the "self-learning with helpers" delivery mode, everyone in the training team is a helper and 
 there are no instructors per se.
 You may have more experienced helpers delivering introductions to the workshop and sections.
-Contact the course authors for the section intro slides you can reuse and you can also find slides for each 
+Contact the course authors for section intro slides you can reuse, and you can also find slides for each 
 section in the course repository (for intructor-led delivery mode).
 
 Roles and responsibilities of helpers include: