Rename tutte_coxeter_36e -> tutte_coxeter_36k

It was e for edge, now k for keys. Why? I'm toying with ideas
for a full Tutte Coxeter graph (30 vertices/nodes/GPIO pins,
45 edges/switches) incuding 42 keys plus a rotary encoder
(these act like three switches: clockwise, anticlockwise,
press - so 42 edges for normal keys + 3 edges for a rotary
encoder gives 42 + 3 = 45 edges) which could be named with
suffix 43ke.

qmk.json

@@ -1,6 +1,6 @@
 {
     "userspace_version": "1.1",
     "build_targets": [
-        ["tutte_coxeter_36e", "default"]
+        ["tutte_coxeter_36k", "default"]
     ]
 }