Fix odd error message in 2020/endoh3/run_clock.sh

When running the script from the shell one would get:

    -bash: ./run_clock.sh: cannot execute: required file not found

and when running from within vim one would get:

    /bin/bash: ./run_clock.sh: /usr/bin/end: bad interpreter: No such file or directory

The problem was that the script had in the shebang /usr/bin/end not
/usr/bin/env.

When I updated the thanks-for-fixes.md file for this I also noted that I
reported to Yusuke in the preview period back in 2020 that in some
systems (it was macOS) due possibly to a timing issue there the script
(which the judges had added) did not work properly with 'make' so Yusuke
changed it to compile the code directly.

2020/endoh3/run_clock.sh

@@ -1,7 +1,8 @@
-#!/usr/bin/end bash
+#!/usr/bin/env bash
+
 while true; do
-cc -std=c11 -Wall -Wextra -pedantic -O3 clock.c -o clock
-clear
-./clock | tee clock.c
-sleep 5
+    cc -std=c11 -Wall -Wextra -pedantic -O3 clock.c -o clock
+    clear
+    ./clock | tee clock.c
+    sleep 5
 done