Replace "signed" 16-bit PNG comment with "unsigned"

spec/2d-gridded-coverage/clause_6_informative_text.adoc

@@ -24,7 +24,7 @@ The definition used in this standard accommodates any reference surface, includi
 
 === Storage formats and grid cell values
 
-This extension to the OGC GeoPackage Encoding Standard leverages the existing structure for raster tiles as defined in GeoPackage Clause 2.2 using PNG (16-bit, signed integer) and TIFF (32-bit, float) files as the container for the grid values, such as elevation. The data producer has the responsibility to ensure that the intended recipient is able to read GeoPackages that contain a grid-regular coverage encoding (PNG or TIFF). Please note that using multiple zoom levels for a grid-regular coverage is purely optional. There is no requirement that grid-regular tiles have to be in a full pyramid. Clause 2.2 specifies how each zoom level is encoded. There may be only one zoom level.
+This extension to the OGC GeoPackage Encoding Standard leverages the existing structure for raster tiles as defined in GeoPackage Clause 2.2 using PNG (16-bit, unsigned integer) and TIFF (32-bit, float) files as the container for the grid values, such as elevation. The data producer has the responsibility to ensure that the intended recipient is able to read GeoPackages that contain a grid-regular coverage encoding (PNG or TIFF). Please note that using multiple zoom levels for a grid-regular coverage is purely optional. There is no requirement that grid-regular tiles have to be in a full pyramid. Clause 2.2 specifies how each zoom level is encoded. There may be only one zoom level.
 
 Clause 7.1.1.2 and Table 1 Ancillary Coverage Data Definition define how the application to specify how the value of a grid cell is assigned. Obviously, if we are dealing with a _continuous_ gridded coverage, values between grid cell center points can be interpolated, such as for contouring.