Uniformize usage() and extra_usage() message ending for commands helper. (#8268)

# Description

Working on uniformizing the ending messages regarding methods usage()
and extra_usage(). This is related to the issue
https://github.com/nushell/nushell/issues/5066 after discussing it with
@jntrnr

# User-Facing Changes

None.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/strings/encode_decode/decode_base64.rs

@@ -40,7 +40,7 @@ impl Command for DecodeBase64 {
     }
 
     fn usage(&self) -> &str {
-        "Base64 decode a value"
+        "Base64 decode a value."
     }
 
     fn extra_usage(&self) -> &str {

crates/nu-command/src/strings/encode_decode/encode_base64.rs

@@ -36,7 +36,7 @@ impl Command for EncodeBase64 {
     }
 
     fn usage(&self) -> &str {
-        "Encode a string or binary value using Base64"
+        "Encode a string or binary value using Base64."
     }
 
     fn examples(&self) -> Vec<Example> {