add space for buggy kernel warning

Summary:
add the missing space
Closes https://github.com/facebook/rocksdb/pull/2150

Differential Revision: D4880696

Pulled By: lightmark

fbshipit-source-id: a4e0ad6a8ea45d6469d3f6c8514fdeb4cf10aaf5

env/io_posix.cc

@@ -777,8 +777,8 @@ Status PosixWritableFile::Close() {
             file_stats.st_blksize !=
         file_stats.st_blocks / (file_stats.st_blksize / 512)) {
       fprintf(stderr,
-              "WARNING: Your kernel is buggy (<= 4.0.x) and does not free"
-              "preallocated blocks on truncate. Hacking around it, but you"
+              "WARNING: Your kernel may be buggy (<= 4.0.x) and does not free "
+              "preallocated blocks on truncate. Hacking around it, but you "
               "should upgrade!\n");
       buggy = true;
     }