scripts/dtc: dtx_diff - make help text formatting consistent

None of the help texts use capitalization, except the one for the -T
option.  Drop the capitalization for consistency.
Split the single long line that doesn't fit in 80 characters.
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200819124709.20401-1-geert+renesas@glider.be

scripts/dtc/dtx_diff

@@ -29,7 +29,8 @@ Usage:
        -s SRCTREE   linux kernel source tree is at path SRCTREE
                         (default is current directory)
        -S           linux kernel source tree is at root of current git repo
-       -T           Annotate output .dts with input source file and line (-T -T for more details)
+       -T           annotate output .dts with input source file and line
+                        (-T -T for more details)
        -u           unsorted, do not sort DTx