slog-handler-guide: remove doubled word

Change-Id: I2fc1ead51b4da07a56201ceab4abd4f9e0643294
GitHub-Last-Rev: bca7c8e
GitHub-Pull-Request: #38
Reviewed-on: https://go-review.googlesource.com/c/example/+/662795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>

Author: bitlux

slog-handler-guide/README.md

@@ -853,7 +853,7 @@ impossible to inspect a system, as is typically the case with a production
 server, logs provide the most detailed way to understand its behavior.
 Therefore, your handler should be robust to bad input.
 
-For example, the usual advice when when a function discovers a problem,
+For example, the usual advice when a function discovers a problem,
 like an invalid argument, is to panic or return an error.
 The built-in handlers do not follow that advice.
 Few things are more frustrating than being unable to debug a problem that