[MINOR] Minor style fix in SparkR

`dev/lintr-r` passes on my machine now

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #8601 from shivaram/sparkr-style-fix.

R/pkg/R/deserialize.R

@@ -60,7 +60,7 @@ readString <- function(con) {
   raw <- readBin(con, raw(), stringLen, endian = "big")
   string <- rawToChar(raw)
   Encoding(string) <- "UTF-8"
-  string 
+  string
 }
 
 readInt <- function(con) {