Fix warning admonition in std/streams (#25564)

The rest of the body must be indented in order to fall under the warning
admonition. Right now, only the first part of the warning is inside the
admonition, see [std/streams](https://nim-lang.org/docs/streams.html).

Author: hovsater

lib/pure/streams.nim

@@ -16,9 +16,9 @@
 ## stream interface.
 ##
 ## .. warning:: Due to the use of `pointer`, the `readData`, `peekData` and
-## `writeData` interfaces are not available on the compile-time VM, and must
-## be cast from a `ptr string` on the JS backend. However, `readDataStr` is
-## available generally in place of `readData`.
+##   `writeData` interfaces are not available on the compile-time VM, and must
+##   be cast from a `ptr string` on the JS backend. However, `readDataStr` is
+##   available generally in place of `readData`.
 ##
 ## Basic usage
 ## ===========