doc: fix link warnings

docs/markdown/External-commands.md

@@ -23,7 +23,7 @@ run_command('command', 'arg1', 'arg2', env: {'FOO': 'bar'})
 ```
 
 Since 0.50.0, you can also pass the command
-[`environment`](Reference-manual.html#environment-object) object:
+[`environment`](Reference-manual.md#environment-object) object:
 
 ```meson
 env = environment()

docs/markdown/Release-notes-for-0.50.0.md

@@ -197,8 +197,8 @@ dependency tracking does not work.
 
 ## `run_command()` accepts `env` kwarg
 
-You can pass [`environment`](Reference-manual.html#environment-object)
-object to [`run_command`](Reference-manual.html#run-command), just
+You can pass [`environment`](Reference-manual.md#environment-object)
+object to [`run_command`](Reference-manual.md#run-command), just
 like to `test`:
 
 ```meson

docs/markdown/Unit-tests.md

@@ -32,7 +32,7 @@ By default, environment variable
 [`MALLOC_PERTURB_`](http://man7.org/linux/man-pages/man3/mallopt.3.html)
 is set to a random value between 1..255. This can help find memory
 leaks on configurations using glibc, including with non-GCC compilers.
-This feature can be disabled as discussed in [test()](./Reference-manual#test).
+This feature can be disabled as discussed in [test()](Reference-manual.md#test).
 
 ## Coverage