fw_env.config: Correct "fw_saveenv" to "fw_setenv".

Fix a comment in the fw_env.config file, no functional change.
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>

tools/env/fw_env.config

-# Configuration file for fw_(printenv/saveenv) utility.
+# Configuration file for fw_(printenv/setenv) utility.
 # Up to two entries are valid, in this case the redundant
 # environment sector is assumed present.
 # Notice, that the "Number of sectors" is ignored on NOR and SPI-dataflash.