Docs: correct the type for BMP x/y density (#4695)

These values are read & written as integers
and don't work unless they are accessed using int accessors.

m_dib_header.hres = m_spec.get_int_attribute("XResolution");

Correct documentation.

BMP uses integer access:
- `./src/bmp.imageio/bmpinput.cpp:`, `m_spec.attribute("XResolution",
(int)m_dib_header.hres);`
- `./src/bmp.imageio/bmpoutput.cpp`, `m_dib_header.hres =
m_spec.get_int_attribute("XResolution");`

Signed-off-by: Campbell Barton <ideasman42@gmail.com>

Author: ideasman42

src/doc/builtinplugins.rst

@@ -42,10 +42,10 @@ tiles.
      - The compression of the BMP file (``"rle4"`` or ``"rle8"``, if
        RLE compression is used).
    * - ``XResolution``
-     - float
+     - int
      - hres
    * - ``YResolution``
-     - float
+     - int
      - vres
    * - ``ResolutionUnit``
      - string