Doc: ARM: Fix a typo in clksrc-change-registers.awk

This patch fix a spelling typo found in clksrc-change-registers.awk.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arm/Samsung/clksrc-change-registers.awk

@@ -41,7 +41,7 @@ function find_length(f)
     else if (f ~ /0xf/)
 	return 4
 
-    printf "unknown legnth " f "\n" > "/dev/stderr"
+    printf "unknown length " f "\n" > "/dev/stderr"
     exit
 }