README: fix formatting

README.md

@@ -1,23 +1,26 @@
 Configure Nix channels:
-``
+
+```shell
 nix-channel --add https://nixbld.m-labs.hk/channel/custom/artiq/fast-beta/artiq-fast
 nix-channel --update
-``
+```
 
 Pure build with Nix:
-``
+
+```shell
 nix-build -A zc706-jtag
 ./remote_run.sh
-``
+```
 
 Impure incremental build:
-``
+
+```shell
 nix-shell
 cd src
 ./zc706.py -g  # build gateware
 make           # build firmware
 cd ..
 ./remote_run.sh -i
-``
+```
 
 The impure build process can also be used on non-Nix systems.