Base: Fix sendfd's man page return section

Author: https://github.com/kevmeyer
Commit: https://github.com/SerenityOS/serenity/commit/ebbcef926a0
Pull-request: https://github.com/SerenityOS/serenity/pull/2643

Base/usr/share/man/man2/sendfd.md

@@ -18,7 +18,7 @@ File descriptors are sent out-of-band and do not affect the regular data streams
 
 ## Return value
 
-If a file descriptor is successfully received, it is returned as a positive integer. Otherwise, -1 is returned and `errno` is set to indicate the error.
+If a file descriptor is successfully sent, `sendfd()` returns 0. Otherwise, -1 is returned and `errno` is set to indicate the error.
 
 ## Errors