Fix typo in LCD mode timing description (#464)

* fix typo in LCD mode timing description

the microsecond estimates for mode timings didn't line up. presumably HBlank is
intended to pad times for each line at a consistent 376 dots, and adding
minimum mode 3 + maximum mode 0 equals maximum mode 3 + minimum mode 0 - 376 
dots either way. but the microsecond estimates didn't match: 60 + 20 = 80µs vs
40 + 49 = 89µs.

presumably the correct µs estimates for mode 3 are "(40 to 69µs)":
>> 291 / (2 ** 22) * 1000 * 1000
> 69.38 (microseconds)

correcting this value makes the min/max sums of mode 3/mode 0 times
consistently 89µs, which is almost the same as the time for 376 dots:
>> 376 / (2 ** 22) * 1000 * 1000
> 89.65

* remove wall-clock timings from mode timing table

while these times are mostly correct, readers typically will be working in
terms of dots (hardware clock cycles) rather than with respect to wall clock
times. in the case that someone needs timings in terms of seconds, these
numbers were not sufficiently precise anyway: rounding to microseconds
introduces error up to 1.4% (85 dots -> ~20.2656 microseconds -> round to 20
-> off by ~0.2656 microseconds for 1.4%).

* Update src/STAT.md

Co-authored-by: Eldred Habert <[email protected]>

Author: iximeow

src/STAT.md

@@ -51,10 +51,10 @@ to the CPU: writes are ignored, and reads return garbage values (usually $FF).
 
 Mode | Action                                                      | Duration                                                           | Accessible video memory
 -----|------------------------------------------------------------------|--------------------------------------------------------------------|-------------------------
-  2  | Searching OAM for OBJs whose Y coordinate overlap this line | 80 dots (19 µs)                                                    | VRAM, CGB palettes
-  3  | Reading OAM and VRAM to generate the picture                | 168 to 291 dots (40 to 60 µs) depending on sprite count            | None
-  0  | Nothing (HBlank)                               | 85 to 208 dots (20 to 49 µs) depending on previous mode 3 duration | VRAM, OAM, CGB palettes
-  1  | Nothing (VBlank)                                 | 4560 dots (1087 µs, 10 scanlines)                                  | VRAM, OAM, CGB palettes
+  2  | Searching OAM for OBJs whose Y coordinate overlap this line | 80 dots                                               | VRAM, CGB palettes
+  3  | Reading OAM and VRAM to generate the picture                | 168 to 291 dots, depending on sprite count            | None
+  0  | Nothing (HBlank)                                            | 85 to 208 dots, depending on previous mode 3 duration | VRAM, OAM, CGB palettes
+  1  | Nothing (VBlank)                                            | 4560 dots (10 scanlines)                              | VRAM, OAM, CGB palettes
 
 ## Properties of STAT modes