doc: fixes default shell in child_process.md

Wrapped `%COMSPEC%` with percent signs.

Fixes: nodejs#14156

doc/api/child_process.md

@@ -1286,7 +1286,7 @@ On Windows, command line parsing should be compatible with `'cmd.exe'`.
 
 ## Default Windows Shell
 
-Although Microsoft specifies `%COMSPEC` must contain the path to
+Although Microsoft specifies `%COMSPEC%` must contain the path to
 `'cmd.exe'` in the root environment, child processes are not always subject to
 the same requirement. Thus, in `child_process` functions where a shell can be 
 spawned, `'cmd.exe'` is used as a fallback if `process.env.ComSpec` is