fix typo in readme
auto merge of #15267 : ruud-v-a/rust/patch-1, r=huonw

[This commit](https://github.com/rust-lang/rust/commit/25fe2cadb10db1a54cefbd1520708d4397874bc3#diff-04c6e90faac2675aa89e2176d2eec7d8R57) forgot to remove one line.