doc: update code language flag for internal doc

Use `bash` instead of `shell` for code language flag in
doc/guides/maintaining-ngtcp2-nghttp3.md to conform with our other docs
and upcoming lint requirements.

PR-URL: #33852
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>

doc/guides/maintaining-ngtcp2-nghttp3.md

@@ -10,7 +10,7 @@ updated together.
 ## Updating ngtcp2
 
 Update ngtcp2:
-```shell
+```bash
 git clone https://github.com/ngtcp2/ngtcp2
 cd ngtcp2
 autoreconf -i
@@ -29,7 +29,7 @@ Check that Node.js still builds and tests.
 ## Updating nghttp3
 
 Update nghttp3:
-```shell
+```bash
 git clone https://github.com/ngtcp2/nghttp3
 cd nghttp3
 autoreconf -i