doc: use backticks around file names in README.md

Use backticks around `SHASUM256.txt` etc. in README.md.

PR-URL: #23299
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

README.md

@@ -116,10 +116,10 @@ _docs_ subdirectory. Version-specific documentation is also at
 
 ### Verifying Binaries
 
-Download directories contain a SHASUMS256.txt file with SHA checksums for the
+Download directories contain a `SHASUMS256.txt` file with SHA checksums for the
 files.
 
-To download SHASUMS256.txt using `curl`:
+To download `SHASUMS256.txt` using `curl`:
 
 ```console
 $ curl -O https://nodejs.org/dist/vx.y.z/SHASUMS256.txt
@@ -132,9 +132,9 @@ it through `sha256sum` with a command such as:
 $ grep node-vx.y.z.tar.gz SHASUMS256.txt | sha256sum -c -
 ```
 
-For Current and LTS, the GPG detached signature of SHASUMS256.txt is in
-SHASUMS256.txt.sig. You can use it with `gpg` to verify the integrity of
-SHASUM256.txt. You will first need to import all the GPG keys of individuals
+For Current and LTS, the GPG detached signature of `SHASUMS256.txt` is in
+`SHASUMS256.txt.sig`. You can use it with `gpg` to verify the integrity of
+`SHASUM256.txt`. You will first need to import all the GPG keys of individuals
 authorized to create releases. They are at the bottom of this README under
 [Release Team](#release-team). To import the keys:
 
@@ -144,7 +144,7 @@ $ gpg --keyserver pool.sks-keyservers.net --recv-keys DD8F2338BAE7501E3DD5AC78C2
 
 See the bottom of this README for a full script to import active release keys.
 
-Next, download the SHASUMS256.txt.sig for the release:
+Next, download the `SHASUMS256.txt.sig` for the release:
 
 ```console
 $ curl -O https://nodejs.org/dist/vx.y.z/SHASUMS256.txt.sig