docs: Fix SECRET_KEY variable description (outline#2229)

Updated the description of the SECRET_KEY variable in the .env.sample
file to clarify that the key needs to be 32 bytes long and hex-encoded.
The previous description of "32 character hexadecimal" was confusing
as it left open the possibility of a hex-encoded 16-byte key.

.env.sample

@@ -8,8 +8,7 @@
 
 # –––––––––––––––– REQUIRED ––––––––––––––––
 
-# Generate a unique 32 character hexadecimal key. The format is important as this
-# value is fed directly into encryption libraries. You should use `openssl rand -hex 32`
+# Generate a hex-encoded 32-byte random key. You should use `openssl rand -hex 32`
 # in your terminal to generate a random value.
 SECRET_KEY=generate_a_new_key