Fix case of "AsciiDoc"

This matches how it is written on the README.

doc/hyperfine.1

@@ -142,7 +142,7 @@ Set the time \fIunit\fP to be used. Default: second. Possible values: millisecon
 .HP
 \fB\-\-export\-asciidoc\fR \fIfile\fP 
 .IP
-Export the timing summary statistics as an Asciidoc table to the given \fIfile\fP.
+Export the timing summary statistics as an AsciiDoc table to the given \fIfile\fP.
 .HP
 \fB\-\-export\-csv\fR \fIfile\fP
 .IP

src/hyperfine/app.rs

@@ -189,7 +189,7 @@ fn build_app() -> App<'static, 'static> {
                 .long("export-asciidoc")
                 .takes_value(true)
                 .value_name("FILE")
-                .help("Export the timing summary statistics as an Asciidoc table to the given FILE."),
+                .help("Export the timing summary statistics as an AsciiDoc table to the given FILE."),
         )
         .arg(
             Arg::with_name("export-csv")