tools: env: update lock path in README

Commit aeb40f1 ("tools: env: use /run to store lockfile") updated the
path to the lockfile but did not update the documentation to match.

Use the new path in the documentation.

Fixes: aeb40f1 ("tools: env: use /run to store lockfile")
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools/env/README

@@ -59,5 +59,5 @@ this environment instance. On NAND this is used to limit the range
 within which bad blocks are skipped, on NOR it is not used.
 
 To prevent losing changes to the environment and to prevent confusing the MTD
-drivers, a lock file at /var/lock/fw_printenv.lock is used to serialize access
+drivers, a lock file at /run/fw_printenv.lock is used to serialize access
 to the environment.