doc: netlink: Fix generated .rst for multi-line docs

Fix the newline replacement in ynl-gen-rst.py to put spaces between
concatenated lines. This fixes the broken doc string formatting.

See the dpll docs for an example of broken concatenation:

https://docs.kernel.org/6.9/networking/netlink_spec/dpll.html#lock-statusSigned-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240528140652.9445-2-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-rst.py

@@ -49,7 +49,7 @@ def inline(text: str) -> str:
 def sanitize(text: str) -> str:
     """Remove newlines and multiple spaces"""
     # This is useful for some fields that are spread across multiple lines
-    return str(text).replace("\n", "").strip()
+    return str(text).replace("\n", " ").strip()
 
 
 def rst_fields(key: str, value: str, level: int = 0) -> str: