kbuild: cc-cross-prefix spelling

Spelling fixes for cc-cross-prefix documentation
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/makefiles.txt

@@ -519,17 +519,17 @@ more details, with real examples.
 	to the user why it stops.
 
     cc-cross-prefix
-	cc-cross-prefix is used to check if there exist a $(CC) in path with
+	cc-cross-prefix is used to check if there exists a $(CC) in path with
 	one of the listed prefixes. The first prefix where there exist a
 	prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
 	then nothing is returned.
 	Additional prefixes are separated by a single space in the
 	call of cc-cross-prefix.
-	This functionality is usefull for architecture Makefile that try
-	to set CROSS_COMPILE to well know values but may have several
+	This functionality is useful for architecture Makefiles that try
+	to set CROSS_COMPILE to well-known values but may have several
 	values to select between.
-	It is recommended only to try to set CROSS_COMPILE is it is a cross
-	build (host arch is different from target arch). And is CROSS_COMPILE
+	It is recommended only to try to set CROSS_COMPILE if it is a cross
+	build (host arch is different from target arch). And if CROSS_COMPILE
 	is already set then leave it with the old value.
 
 	Example: