fix grammar in documentation: Remove 'of' from 'Both of docker images…

…' and add 'is' to 'This image manually maintained'

ci/docker-rust/README.md

@@ -1,9 +1,9 @@
 Docker image containing rust and some preinstalled packages used in CI.
 
 NOTE: Recreate rust-nightly docker image after this when updating the stable rust
-version! Both of docker images must be updated in tandem.
+version! Both docker images must be updated in tandem.
 
-This image manually maintained:
+This image is manually maintained:
 1. Edit `Dockerfile` to match the desired rust version
 1. Run `docker login` to enable pushing images to Docker Hub, if you're authorized.
 1. Run `./build.sh` to publish the new image, if you are a member of the [Solana