doc: android/boot-image: invalid C declaration

make htmldocs results in an error:

doc/android/boot-image.rst:33:
WARNING: Unparseable C cross-reference: 'struct andr_img_hdr'
Invalid C declaration: Expected identifier in nested name, got keyword:
struct [error at 6]

Follow the style prescribed in
https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#highlights-and-cross-references



Add missing definite article.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>