Rollup merge of #64606 - adrianheine:patch-1, r=sfackler
Rollup merge of #64606 - adrianheine:patch-1, r=sfackler

Remove unnecessary `mut` in doc example

graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust