Errorprone: Fix update.sh syntax issue

There was a space missing.

Test: external/error_prone/update.sh
Change-Id: I0c6952e5c75dd488e1b31b3cb9bb6bed5725c366
diff --git a/update.sh b/update.sh
index 7db048b..1d3061b 100755
--- a/update.sh
+++ b/update.sh
@@ -1,7 +1,7 @@
 #!/bin/bash
 # Force stop on first error.
 set -e
-if [ $# -ne 2 -a $# -ne 3]; then
+if [ $# -ne 2 -a $# -ne 3 ]; then
     echo "$0 <error prone version> <error prone javac version> [checkerframework version]" >&2
     exit 1;
 fi