Update _extras/guide.md

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

Author: anenadic

_extras/guide.md

@@ -88,7 +88,7 @@ when students start with group exercises.
 
 ### Helpers Roles and Responsibilities
 At the workshop, when using the "self-learning with helpers" delivery mode, everyone in the training team is a helper and 
-there are no insturctors per se.
+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 
 section in the course repository (for intructor-led delivery mode).