docs: customize-toolchain: fix typo

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/manual/customize-toolchain.txt

@@ -28,7 +28,7 @@ However, it allows to tune major settings, such as:
 
 * Binutils, GCC, Gdb and toolchain options
 
-This is directly avaiblable after selecting the +Buildroot toolchain+ type in
+This is directly available after selecting the +Buildroot toolchain+ type in
 the menu +Toolchain+.
 
 Using the Crosstool-NG backend