Change "Returns" description

Change read to write as this function should return the number of bytes in the output buffer if I'm getting this right.

Author: irieger

Language/Functions/Communication/Serial/availableForWrite.adoc

@@ -35,7 +35,7 @@ Nothing
 
 [float]
 === Returns
-The number of bytes available to read .
+The number of bytes available to write.
 --
 // OVERVIEW SECTION ENDS