slog-handler-guide/README.md: fix word duplication

Change-Id: Id17b4acd73e237813292a63112b30e915d652bd4
GitHub-Last-Rev: 2052832a0d
GitHub-Pull-Request: golang/example#29
Reviewed-on: https://go-review.googlesource.com/c/example/+/550196
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Run-TryBot: qiulaidongfeng <2645477756@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Bypass: Jonathan Amsterdam <jba@google.com>

slog-handler-guide/README.md

@@ -67,7 +67,7 @@ original remains unchanged.
 All subsequent output from `logger` will include those attributes.
 A logger's `With` method calls its handler's `WithAttrs` method.
 
-The `WithGroup` method is used to avoid avoid key collisions in large programs
+The `WithGroup` method is used to avoid key collisions in large programs
 by establishing separate namespaces.
 This call creates a new `Logger` value with a group named "g":