doc: Americanise spelling of wilful

Fixes #13660

Change-Id: I05bcb4efcb865192a1ef6756e9dccef83505934c
Reviewed-on: https://go-review.googlesource.com/17990
Reviewed-by: Chris Broadfoot <cbro@golang.org>
diff --git a/doc/conduct.html b/doc/conduct.html
index 73b68f0..ce824a1 100644
--- a/doc/conduct.html
+++ b/doc/conduct.html
@@ -159,7 +159,7 @@
 If you conduct yourself in a way that is explicitly forbidden by the CoC,
 you will be warned and asked to stop.
 If you do not stop, you will be removed from our community spaces temporarily.
-Repeated, wilful breaches of the CoC will result in a permanent ban.
+Repeated, willful breaches of the CoC will result in a permanent ban.
 </p>
 
 <p>