Update map-syntax text in manpage to match --help

assets/manual/bat.1.in

@@ -123,6 +123,7 @@ Map a glob pattern to an existing syntax name. The glob pattern is matched on th
 path and the filename. For example, to highlight *.build files with the Python syntax,
 use -m '*.build:Python'. To highlight files named '.myignore' with the Git Ignore
 syntax, use -m '.myignore:Git Ignore'.
+Note that the right-hand side is the *name* of the syntax, not a file extension.
 .HP
 \fB\-\-theme\fR <theme>
 .IP