docs(ironvars): correct allowed chars in keys

docs/Ironvars.md

@@ -1,7 +1,7 @@
 Ironvars are runtime variables that can be referenced in several places in your config, 
 then set using the IPC server (such as via the CLI) using the `set` command.
 
-Any UTF-8 string *without whitespace* is a valid key. 
+Keys can consist of alphanumeric characters, `-` and `_` only.
 Any UTF-8 string is a valid value.
 
 Reference values using `#my_variable`. These update as soon as the value changes.