io.backend: fix docs to show nanoseconds instead of micros.

basis/io/backend/unix/unix.factor

@@ -135,7 +135,7 @@ M: unix (wait-to-write) ( port -- )
     dup handle>> dup check-disposed drain
     dup [ wait-for-port ] [ 2drop ] if ;
 
-M: unix io-multiplex ( ms/f -- )
+M: unix io-multiplex ( nanos -- )
     mx get-global wait-for-events ;
 
 ! On Unix, you're not supposed to set stdin to non-blocking

core/io/backend/backend-docs.factor

@@ -2,8 +2,8 @@ USING: help.markup help.syntax io io.backend strings
 byte-arrays ;
 
 HELP: io-multiplex
-{ $values { "us" "a non-negative integer" } }
-{ $contract "Waits up to " { $snippet "us" } " microseconds for pending I/O requests to complete." } ;
+{ $values { "nanos" "a non-negative integer" } }
+{ $contract "Waits up to " { $snippet "nanos" } " nanoseconds for pending I/O requests to complete." } ;
 
 HELP: init-io
 { $contract "Initializes the I/O system. Called on startup." } ;

core/io/backend/backend.factor

@@ -19,7 +19,7 @@ HOOK: init-stdio io-backend ( -- )
     [ utf8 <encoder> output-stream set-global ]
     [ utf8 <encoder> error-stream set-global ] tri* ;
 
-HOOK: io-multiplex io-backend ( us -- )
+HOOK: io-multiplex io-backend ( nanos -- )
 
 HOOK: normalize-directory io-backend ( path -- path' )