Fix typos in /Documentation : 'U-Z'

This patch fixes typos in various Documentation txts. The patch addresses some
+words starting with the letters 'U-Z'.

Looks like I made it through the alphabet...just in time to start over again
+too!  Maybe I can fit more profound fixes into the next round...?  Time will
+tell. :)

Signed-off-by: Matt LaPlante <kernel1@cyberdogtech.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

Documentation/filesystems/spufs.txt

@@ -210,7 +210,7 @@ FILES
    /signal2
        The two signal notification channels of an SPU.  These  are  read-write
        files  that  operate  on  a 32 bit word.  Writing to one of these files
-       triggers an interrupt on the SPU. The  value  writting  to  the  signal
+       triggers an interrupt on the SPU.  The  value  written  to  the  signal
        files can be read from the SPU through a channel read or from host user
        space through the file.  After the value has been read by the  SPU,  it
        is  reset  to zero.  The possible operations on an open signal1 or sig-