Fix up the docs warning detected by the vale Acronym rule

doc/.vale/gitlab/Acronyms.yml

@@ -116,6 +116,7 @@ exceptions:
   - PEM
   - PEP
   - PGP
+  - PKCS
   - PHP
   - PNG
   - POSIX

doc/administration/smime_signing_email.md

@@ -69,16 +69,16 @@ The key needs to be readable by the GitLab system user (`git` by default).
 
 The key needs to be readable by the GitLab system user (`git` by default).
 
-### How to convert S/MIME PKCS#12 / PFX format to PEM encoding
+### How to convert S/MIME PKCS #12 / PFX format to PEM encoding
 
-Typically S/MIME certificates are handled in binary PKCS#12 format (`.pfx` or `.p12`
+Typically S/MIME certificates are handled in binary PKCS #12 format (`.pfx` or `.p12`
 extensions), which contain the following in a single encrypted file:
 
 - Public certificate
 - Intermediate certificates (if any)
 - Private key
 
-To export the required files in PEM encoding from the PKCS#12 file, the
+To export the required files in PEM encoding from the PKCS #12 file, the
 `openssl` command can be used:
 
 ```shell