Typo fix: Returnin => Returning (#67)

Co-authored-by: Herwin <herwinw@users.noreply.github.com>

Author: herwinw

4.0.md

@@ -141,7 +141,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
   p u
   #<User:0x000071d1b04b07a0 @first="Jane">
 
-  # Returnin `nil` from the method means "use the default rendering"
+  # Returning `nil` from the method means "use the default rendering"
   class FullUser < User
     def instance_variables_to_inspect = nil
   end

_src/4.0.md

@@ -141,7 +141,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
   p u
   #<User:0x000071d1b04b07a0 @first="Jane">
 
-  # Returnin `nil` from the method means "use the default rendering"
+  # Returning `nil` from the method means "use the default rendering"
   class FullUser < User
     def instance_variables_to_inspect = nil
   end