docs: unify code and shell cmd blocks

docs/code_contribution_guidelines.md

@@ -226,7 +226,7 @@ func DeriveRevocationPubkey(commitPubKey *btcec.PublicKey,
   obvious<br /><br />
 
 **WRONG**
-```Go
+```go
 // return err if amt is less than 546
 if amt < 546 {
 	return err
@@ -256,7 +256,7 @@ being provided here.
 
 Here’s a model Git commit message:
 
-```
+```text
 Short (50 chars or less) summary of changes
 
 More detailed explanatory text, if necessary.  Wrap it to about 72
@@ -506,13 +506,13 @@ with out testing infrastructure, or simply make a PR into `lnd` that will build
 without any further work, the `go.mod` and `go.sum` files will need to be
 updated. Luckily, the `go mod` command has a handy tool to do this
 automatically so developers don't need to manually edit the `go.mod` file:
-```
- go mod edit -replace=IMPORT-PATH-IN-LND@LND-VERSION=DEV-FORK-IMPORT-PATH@DEV-FORK-VERSION
+```shell
+⛰  go mod edit -replace=IMPORT-PATH-IN-LND@LND-VERSION=DEV-FORK-IMPORT-PATH@DEV-FORK-VERSION
 ```
 
 Here's an example replacing the `lightning-onion` version checked into `lnd` with a version in roasbeef's fork:
-```
- go mod edit -replace=github.com/lightningnetwork/lightning-onion@v0.0.0-20180605012408-ac4d9da8f1d6=github.com/roasbeef/lightning-onion@2e5ae87696046298365ab43bcd1cf3a7a1d69695
+```shell
+⛰  go mod edit -replace=github.com/lightningnetwork/lightning-onion@v0.0.0-20180605012408-ac4d9da8f1d6=github.com/roasbeef/lightning-onion@2e5ae87696046298365ab43bcd1cf3a7a1d69695
 ```
 
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