doc: fix typo in test/common/README.md

Corrects the misspelling of "unmask" to "umask"

PR-URL: #59180
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: ri7116

test/common/README.md

@@ -14,7 +14,7 @@ several other tasks:
   don't accidentally pollute the global namespace.
 
 * Some tests assume a default umask of `0o022`. To enforce this assumption,
-  the common module sets the unmask at startup. Tests that require a
+  the common module sets the umask at startup. Tests that require a
   different umask can override this setting after loading the module.
 
 * Some tests specify runtime flags (example, `--expose-internals`) via a