Fix documentation for meta block: string replacement in key from `-…

…` to `_` (#15940)

Signed-off-by: dttung2905 <[email protected]>

website/content/docs/job-specification/meta.mdx

@@ -47,7 +47,8 @@ Meta values are made available inside tasks as [runtime environment variables][e
 
 The "parameters" for the `meta` stanza can be any key-value. The keys and values
 are both of type `string`, but they can be specified as other types. They will
-automatically be converted to strings.
+automatically be converted to strings. Any `-` character existing in the key
+will also be converted to `_`.
 
 ## `meta` Examples