fix: two rivers that were not connected fully (#2422)

## Description:

Literally one pixel was stopping the river
Before
<img width="450" height="405" alt="image"
src="https://github.com/user-attachments/assets/d5af908f-2136-42f7-90cc-655d24372c22"
/>

After
<img width="272" height="238" alt="image"
src="https://github.com/user-attachments/assets/b8f595f3-cf10-4098-89cb-7b5b7f2bc9d1"
/>

before
<img width="222" height="263" alt="image"
src="https://github.com/user-attachments/assets/8a1abd1f-85c8-4673-bf7a-bcc774783e2e"
/>

after
<img width="246" height="217" alt="image"
src="https://github.com/user-attachments/assets/16249b6f-1743-4f04-ad8e-b353b29380de"
/>


## Please complete the following:

- [ X ] I have added screenshots for all UI updates
- [ N/A ] I process any text displayed to the user through
translateText() and I've added it to the en.json file
- [ N/A  ] I have added relevant tests to the test directory
- [ X ] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

@dxtron_28992

---------

Co-authored-by: Evan <evanpelle@gmail.com>

resources/maps/britannia/manifest.json

@@ -1,17 +1,17 @@
 {
   "map": {
     "height": 1396,
-    "num_land_tiles": 933860,
+    "num_land_tiles": 933706,
     "width": 2000
   },
   "map16x": {
     "height": 349,
-    "num_land_tiles": 55046,
+    "num_land_tiles": 55009,
     "width": 500
   },
   "map4x": {
     "height": 698,
-    "num_land_tiles": 228988,
+    "num_land_tiles": 228914,
     "width": 1000
   },
   "name": "Britannia",